Rock fogies are always complaining that singles aren’t what they used to be. Apart from the dance 12-inch, the 45 is a declining form, saved only by indie conservationists and its continuing convenience for the lazier radio stations.

The downfall of the single has also meant the death of the double-sided 45, for the Sixties generation, often the best guide to an act’s creative stamina. Phil Spector might cunningly place bland and featureless instrumentals on his flipsides to guarantee airplay for the A-side but that was just another early sign of that control-freak’s maverick paranoia.

Otherwise, 60s buyers knew it was often worth flipping the disc. ‘Penny Lane’/ ‘Strawberry Fields’ was the Beatles’ sublime bridge between Revolver and Sgt. Pepper’s Lonely Hearts Club Band. Flip the Stones’ ‘You Can’t Always Get What You Want’ and you’d discover ‘No Expectations’, arguably their best country-blues lament. Or by-pass the Who’s ‘Substitute’ in favour of ‘The Ox’, a shuddering instrumental that may be the best example of how Pete Townshend and Keith Moon dynamised each other. Now perhaps only a writer as prolific as Elvis Costello maintains those standards.

And once CD replaced vinyl as the dominant carrier and albums became 60 not 40 minutes, there was even less chance of surplus surprises finishing up on the flipside. Instead any extra tracks added to the package usually only interest the Greater Trainspotted Collector. Reheated live performances and superfluous and emotionally sterile remixes aren’t the stuff of legend.

So how do U2 match up to the 60s’ test? Rather well, actually. Before they started to hit their songwriting stride at The Unforgettable Fire, there’s little to enthuse anyone bar the most specialist fan but thereafter, there’s definitely an album’s worth of material that doesn’t deserve to be left mouldering in the flipside attic.

Of course, these tracks are often less finished but that’s often the point since they haven’t been finessed by the band’s perfectionist tendencies. Listeners get the rawer STS version with Paul Barrett, not what’s later gilded with Daniel Lanois. These cuts can rarely be called corporate; this is U2 unplugged from the demands of fame; quirks in each member’s character are far more audible.

Besides the quirks, there are also clues to U2’s later development. Sometimes you hear the start of ideas with which they only come to terms further down the road. Bono’s Elvis fixation emerges earlier; previous pointers to the jagged pop style of Achtung Baby can be detected. They also – and here critics and champions of the band inevitably divide – document Bono’s own highly individual creative process, searching out the emotion of the song before he settles on a finished lyric. Sometimes the last-minute man has only a verse or a chorus and warbles wordlessly.

Improvisation or indulgence? Well, some of us like a sneak early preview long before the final production has been decided. Besides, taken together, these U2 B-sides document a noisier and far more informal U2, more rock’n’roll cinema verité than the luxurious Hollywood final cut.

Not surprisingly, there’s a few covers of varying merit. Their version of Creedence Clearwater Revival’s ‘Fortunate Son’ will gain more marks for its political sentiments than its straightforward performance but both Bono’s bizarre remodelling of Presley’s ‘Can’t Help Falling In Love With You’ and their 1989 proto-grunge version of Patti Smith’s ‘Barefoot Dancing’ with a hilarious Edge-perfect imitation of Neil Young, don’t deserve to be filed away in obscurity.

I wouldn’t go so far to claim that these B-sides amount to a secret storehouse of must-hear U2 masterpieces. The quality is uneven and, quite often, it’s easy to understand why they didn’t suit the mood of the album of the time. But they are instructive evidence of why U2 have endured.

Both champions and critics of U2 tend, for good or for ill, to seize on their social significance, their business professionalism and their live performances. Somehow U2’s versatility and enduring ability to musically re-invent themselves often goes unnoticed. These B-sides are often the playground where U2 first tried out their new costumes.

There’s no great need to waste paragraphs on their early surplus material. U2’s heavy touring schedule left scant time for fooling around in the studio. Generally their leftovers deserved their demotion to the subs’ bench. Still for completeness’ sake . . .

Of course they did begin with U23, the limited edition and collector’s Grail that was their debut 12-inch single on CBS Ireland that included the first versions of Boy tracks, ‘Out Of Control’ and ‘Stories For Boys’. The dynamics may be more meandering and less compressed than Steve Lillywhite’s production for their debut album but some may find a winning, fresh-faced charm in their first outing.

Thereafter, the interest is mostly academic. ‘Touch’ which backed the lone Martin Hannett production of ‘11 O’Clock Tick Tock’ is a raw sketch, interesting only for the more dryly metallic, nightsilver sound the late Joy Division guru gave them. But the What If question never arose. Hannett and the increasingly Christian U2 could never agree on lifestyle.

‘Things To Make And Do’, an instrumental backing their first Steve Lillywhite single, ‘A Day Without Me’ adds little or nothing to the canon save the realisation that they didn’t always find inspiration on their early sound-check jams. More interesting are ‘J. Swallow’ backing ‘Fire’ and ‘Treasure (Whatever Happened To Pete The Chop)’ on the flip of ‘New Year’s Day’.

The first is another instrumental vamp, a tamer version of the Virgin Prunes with indecipherable vocals that seem – and I claim no infallibility here – just to be repeating “bumpity, bump”. No lessons for Seamus Heaney or Brendan Kennelly in this!

As for ‘Treasure . . .’, it’s a rather nifty pop ditty, an obvious stab at the power-pop song that would raise their radio crossover profile but unfinished without any Edge guitar fills. Only one other track from their juvenalia deserves mention, ‘Trash, Trampoline And Party Girl’ backing ‘Celebration’ later revived through its live version on ‘Under A Blood Red Sky’.

With The Unforgettable Fire, U2 ceased being freshmen in the studio and the extra tracks also show their progress. But it’s also obvious why the two versions of ‘Boomerang’ on the flip of ‘Pride (In The Name Of Love)’ and ‘Bass Trap’ on the 12-inch of ‘The Unforgettable Fire’ didn’t make it to the album since both could be deemed more Eno than U2.

‘Boomerang’ though it shows their first and very tentative interest in the dance remix format, takes too many leaves from the Talking Heads book. And though ‘Bass Trap’ is serenely ambient, Edge playing at being Robert Fripp didn’t then advance the cause of U2’s originality. They wouldn’t have wished to have been so obviously seen as apprentices to Eno’s sorcerer.

‘Love Comes Tumbling’ and ‘Three Sunrises’, also on ‘The Unforgettable Fire’ 12-inch, presumably got ditched for opposite reasons. Both are outside the orbit of the album but both also deserve investigation by any band looking for arcane cover material. ‘Love Comes Tumbling’ is another comparatively Edge-less pop song with an almost French lilt. ‘Three Sunrises’ has massed vocal harmonies and oddly jolting syncopation from both Edge and Larry Mullen that could have ended up as some curious hybrid between Duran Duran and The Cure. Then U2 hesitated to bring such ideas to fruition; today, they’d understand how.

From The Joshua Tree onwards, there’s real meat outside the official menu. A pattern starts to emerge. B-sides become an opportunity for Bono to let off steam, toss ideas around and not be bothered by any demands for impeccable product. Certainly both ‘Luminous Times (Hold On To Love)’ and ‘Walk To The Water’, the two tracks which supported ‘With Or Without You’, deserve wider exposure.

Both are intense emotional exorcisms. ‘Luminous Times . . .’ whose spine is a mournful, piano-led drone, must be his most direct, heartfelt and private love song to his wife. The presumably improvised similes accumulate; she is his “avalanche”, “car-crash”, “thunder”, “speedway” and “slipstream”. Besides, his final confessions – “I love you because I understand God has given me your hand”; and “I love you because I need to, not because I need you” – add an extra dimension to the emotional oscillation of ‘With Or Without You’.

As for ‘Walk To The Water’, it’s his first flirtation with rap. Early Dublin schoolday memories are made flesh as they weren’t on Boy, following some other early flame – her hair’s coloured gold so she can’t be Allie – from Summerhill, down the North Strand onto the Clontarf shore. The mood, if not the instrumentation or the vocal style, isn’t that far from the memories of first loves and adolescent exploration on Astral Weeks.

If these two tracks are special, the five remaining extra cuts from The Joshua Tree-era aren’t slouches either. ‘I Still Haven’t Found (What I’m Looking For)’ came with two additions. ‘Spanish Eyes’ was Texas border rock with unfinished, improvised vocals from Bono about some gypsy love who “comes in colours” straight from the Arthur Lee/Love songbook. ‘Deep In The Heart’ begins from a similar Edge guitar pattern but is slowed down, recontextualised and topped off by a Bono vocal line adjacent to ‘New Year’s Day’.

The various versions of ‘Where The Streets Have No Name’ came with three companions. ‘Silver And Gold’ got its first band version: it’s tough and caustic but doesn’t get under the skin of the song like the later live version on Rattle And Hum. Then there was another oddity, ‘Race Against Time’, U2 again toying with rock-dance grooves to wordless and possibly backwards vocals from Bono. Finally ‘The Sweetest Thing’ was more mock-soul with Bono again in vocal undress, lyrical but pre-lyric, unbothered with studio deadlines, intense with the emotions but economic with the verbals.

The Rattle And Hum surplus brought two new songs. Indeed if they’d pitched both ‘Hallelujah (Here She Comes)’ and ‘A Room At The Heartbreak Hotel’ alongside that album’s other originals and saved all its live tracks for another independent release, who knows if U2 would have suffered the same critical flak? Probably they were riding for a fall but their later development might have been better understood.

As for the two tracks, ‘Hallelujah (Here She Comes)’ with its combination of acoustic guitars and gospel-rock could have been written for the Hot House Flowers while ‘A Room At The Heartbreak Hotel’ is a bad night in Elvis Presley’s nightmares. Edge’s fierce guitar predates Achtung Baby, feuding with the back-up trio of Edna Wright, Maxine and Julia Waters. Jimmy Iovine’s production is compressed to a claustrophobic extent, Bono’s vocals are muffled in a Plastic Ono Band shroud and the obvious model sounds like Phil Spector producing and Jimi Hendrix playing in Graceland. The basic riff may get uprooted in the sonic storm but I wouldn’t quibble with the intention . . .

There were also two new departures. First U2 began a set of eclectic and often abnormal covers, usually concocted with Paul Barrett in STS. Backing ‘When Love Comes To Town’ was Patti Smith’s ‘Dancing Barefoot’. Widely disregarded then, it now merits resurrection if only for some disc-jockey to segue it alongside Pearl Jam, as U2 play grunge in 1989 and Edge peels off an uncannily accurate Neil Young imitation for his solo. If nothing else, this throwaway shows the formulae U2 were rejecting, and why after Rattle And Hum, they’d refuse to be mock-American retro-rockers.

A second set of covers with ‘All I Want Is You’ were even more playfully funny. Their version of the Love Affair’s ‘Everlasting Love’ scuttled through early childhood memories of ‘T.O.T.P.’ and with ‘Unchained Melody’, Bono again reunited Elvis Presley and Phil Spector in the far less reverential backdrop of a Northside karaoke and stag-party.

They also now seriously started to examine the dance mix as the ‘U2 3-D Dance Mix’, unfortunately released only for promotion, had three Rattle And Hum tracks remixed by Louis Silas Jnr. Pride of place went to ‘When Love Comes To Town’, a genuinely successful attempt – long before Primal Scream – to re-adapt blues-rock for the dancefloor. Bono’s vocals gets chopped and squashed, black backing vocals accentuate the gospel tip, Edge gets relegated and Larry and Adam promoted beside B.B. King. But the real coup was inserting audience cheers alongside a Little Richard sermon and gymnastic sax from David Koz in an effort to imagine a new, ecumenical but definitely sanctified and blues-driven coalition between dance and rock communities.

From the vantage point of ’95, these dances mixes now sound specially significant, the real bridge between Rattle And Hum and Achtung Baby. The remodelled ‘Desire’ now seems a crucial blueprint, the first signal of their ambition to re-equip primitive Fifties rock’n’roll rhythms for the Nineties dancefloor. U2 were about to straddle a divide no other rock band dared to cross with conviction. Grunge would be reactionary in its refusal to accept the rhythms revolution of hip-hop. English bands did acknowledge the crisis and look for some Grand Unifying Theory between dance and rock but they lacked the true American R’n’B grit to carry it off with conviction.

‘God Part II’ also got the treatment and is less angry, compressed and a more rhythmically open and diverse version than the album original. Even then, the band had started to imagine the change of roles and responsibilities between Edge and the pair of Adam and Larry that is the most significant difference in Achtung Baby.

Next when ‘The Fly’ was released, Bono and Edge slipped out ‘Alex Descends Into Hell For A Bottle Of Milk’, an excerpt from their soundtrack for the RSC’s theatrical adaptation of Anthony Burgess’ A Clockwork Orange. It starts as classical pastiche with mock-angelic boy sopranos and mock-tubas and concert-hall horns with a typically spangled Edge guitar line before heavy duty hip-hop rhythms lunge in. Was this really the same pair whose previous extra-U2 activity had been writing and recording with the late Roy Orbison?

At this point, it’s not easy to retain both narrative and thematic continuity since both covers and new tracks are dotted through the various 12-inch and CD releases of the Achtung Baby era. On the cover front, ‘One’ tidied up their back-catalogue with a Youth remix of ‘Night And Day’, their contribution to the Red Hot + Blue project and a version of Lou Reed’s ‘Satellite Of Love’ to tie in with Zoo T.V.

Another set of covers, the Stones’ ‘Paint It Black’ and Creedence Clearwater Revival’s ‘Fortunate Son’ fetched up beside the revised and far more convincing single version of ‘Who’s Gonna Ride Your Wild Horses’. Both are straightforward, the only real point of interest being the choice of the anti-Vietnam war Creedence song as a hint that U2 hadn’t totally abandoned their political radicalism.

But another CD version of ‘. . . Wild Horses’ contained one essential curio, Bono wrestling with the ghost of Elvis Presley on ‘Can’t Help Falling In Love’. Just him hammering together some rhythm machines with a keyboard assist from Paul Barrett, at points, this is almost Memphis entering some bizarre acid-jazz deconstruction zone. Bono sings the verse but he’s far more fascinated in gnawing at the bone of the chorus. Sometimes he uses his Fat Lady falsetto, sometimes he goes deep but all his jazzy manoeuvres still find him fumbling for the key to escape from that damned room at the Heartbreak Hotel.

The originals are equally fascinating. ‘Where Did It All Go Wrong’, a companion to ‘Even Better Than The Real Thing’ was a fun bash in the tunnel where The Ramones get recustomised with some neopsychedelic harmony vocal trickery to skid onto and then off the dancefloor. But it’s the two versions of ‘Salome’ and ‘Lady With The Spinning Head’ where U2 really reveal some studio and songwriting secrets.

Both get straight R’n’R and dance treatments. The first and more traditional release of ‘Salome’ backing ‘Even Better Than The Real Thing’ departs from the same station as Them’s ‘Baby Please Don’t Go’ before Edge crunches in with some shattering Keith Richard/ Pete Townshend chords and then Adam’s bass churns up in the mix as if they’re engaged in some private project to sort out how a rejuvenated Faces might sound in the early Nineties. Then they pulled down the house and subjected ‘Salome’ to a Zooromancer remix on a ‘. . . Wild Horses’ release.

As for ‘Lady With The Spinning Head’, its first incarnation appears with ‘One’. Larry ladles on that scrap-happy Madchester beat, Edge carts a curdled guitar-keyboard solo across the screen, a few la-la-la vocal harmonies get chucked into the stew and it all sounds like something they (uncomplicatedly?) threw together on their holidays from Achtung Baby.

But suspicions that U2 are loitering with most definite intent within the vicinity of Primal Scream – anything you can do we can Screamadelica better – are confirmed by the second dance version. This is definitely U2 showing their competitive teeth in an old-fashioned cutting contest, hinting they were present at the Creation long before Bobby Gillespie and Alan McGhee.

Come Zooropa, U2 didn’t have the same surplus to distribute after the official album quota. Touring and recording that album took all their energies. Remixes, a reprise of ‘I’ve Got You Under My Skin’, Bono’s duet with Frank Sinatra and stirring live versions of ‘Bullet The Blue Sky’ and ‘Love Is Blindness’ were the B-side bait.

Bar one exception in the shape of ‘Slow Dancing’, Bono and Edge on acoustic guitars showing off the song they’d written for Willie Nelson. By now, I can only marvel at their genre-hopping on a B-side expedition that’s taken us through Nelson, a RSC soundtrack and the re-insertion of the blues into dance-rock. No other rock act does such shape-shifting on their Star Trek.

Nor do the B-sides exhaust the tracks outside their official album canon. I’m excluding all their individual guest appearences and I still haven’t mentioned either the recent Propaganda fan club release of Melon or their lone live E.P., Wide Awake In America with its fundamental version of Bad. Besides, there are other orphan cameos – the cover of ‘Maggie’s Farm’ on the Self Aid album, Live For Ireland; ‘Christmas (Baby Please Come Home)’ on Jimmy Iovine’s ’87 charity album, A Very Special Christmas on A&M; and finally, their Woody Guthrie cover, ‘Jesus Christ’ on the ’88 Sony compilation, A Vision Shared of various luminaries updating the songs of Guthrie and Leadbelly.

All these are loose ends and some are definitely looser than others. Take your pick. Would a compilation of U2’s dangling bits – selective or otherwise – be a callous mercenary manoeuvre to fleece their fans or a good shepherd’s deed to reintegrate these lost sheep into the fold?

Depending on whether or not you credit their remixes among the lost and found, there’s over 50 tracks in the U2 orphanage. I say at least twenty could fend for themselves. Here be some mistakes but U2 have always been a band who’ve understood how to prosper by learning from their errors. The balance between scrupulous perfectionism and instinct has been one secret of their evolution.

The albums show a U2 that’s very rarely out of control. These B-sides are clues and signposts to their other half.