Many are the wicked instruments of torture that you can clamp to your guitar neck in pursuit of easier playing. Adrian Legg screws, twists and stretches a large cross section of the aforementioned doobries.

CAPOS... an exotic Siberian fruit, a hat worn by Norwegian bull fighters, a device for raising the pitch of your guitar strings en masse by pretending to be a sixth finger. Discuss. Use one side of the desk only.

My erudite and dusty Oxford Dictionary of Music has this to say about those strap/screw/slip on gadgets which fix around your guitar neck: "Capotasto, capo d'astro, capodastro (Italian), capo-dastere (French), capodaster (German). Literally 'head' of the 'touch' or 'feel'."

Percy Scholes then confusingly goes on to describe this as the nut or raised part of the fingerboard which 'touches' the strings, and determines the left hand end of the scale length, pointing out that in the USA the name capotasto is reserved for a moveable type of stop.

So there is the origin of the occasional mix-up over the name. It's the moveable version we're interested in, and know as the capo. Still, just to muddle matters even further, we'll start with the very first item to do the job — the Spanish cejilla, pronounthed with a thoft lithp thuth: "theheelya". Now you may buy one in Ivor Mairants without being gently corrected. Flamencos lisp their c's for added authentithity.

This gadget consists of a small wooden block about the length of a classical fingerboard width, and about 14-15mm wide, with a leather covered 'flat' which presses against the strings. It is held there by a length of nylon attached to one end of the block, taken right round the back of the neck and then fixed to a small fiddle-type peg which fits into a hole on the top of the cejilla and tightens the whole lot up. This is the flamenco purist's capo, but with the addition of some thicker leather to the flat face, and maybe a little shaved off the length, it can work quite well on a cambered fingerboard steel string.

But it can be fiddly to put on, so now we should define how we expect a capo to function. First, it must hold the strings down hard, enough to keep them behind the fret, but not so hard that it bends them out of tune, so the pressure must be variable. Secondly, it should not stick out so much that it gets in the way, and thirdly, it should be easy and quick to fit on.

Also we should consider why we want a capo to function.

There is always the defensive use against the prune who wants to sing 'Freight Train' or 'Southern Belle' in some alien flat key, and Albert Lee used a capo in a UK country band to shift a Jerry Reed open A7 lick into C. In fact, in common with many electric pickers, he'll slip a capo on here and there with no shame. Figure it out, a lot of licks will come no other way.

A capo can clamp down an over-high nut action, and it can increase sustain and clarity on a shot or stillborn cheap acoustic as well as making a Martin sweeter. Some older Yamaha acoustics had a harsh B-string which would back off a bit under a capo.

The inherent piloting problem is that the capo tends to be tougher than your fingers. It can exert a far stronger downward pressure on the strings than the average mitt, and that may produce tuning problems unless you're careful.

Its most positive function is to help set up a series of left hand shapes, with or without unusual tunings, in the key that suits the piece or the singer best, so that work can be transferred to the right hand almost Keith-style. See Jerry Donahue on Jerry Reed in the April issue. Donahue's 'Tokyo' is a good example of the approach. Look out for Bill Keith or Joe Locker to see it working on a five-string, and watch closely how the notes are laid out in stopped and open bunches on the left hand, then selected by the right. If you have the rare luck to see Joe, ask him to play 'Temptation Rag' — a supreme piece of playing.

One of the simplest capos on the market has its roots with black American street musicians who would use a small stick and strong elastic bands to capo a 12-string up to a decent blues-hollerin' pitch. Terry Gould marketed a version or two, and a Frontline one with double elastic retails at 99p. Roughly speaking, it's a 'nail' with the point cut off pushed through the middle of a piece of elastic, then through a rubber tube, with enough far end sticking out for one side of the elastic to be stabbed on to hold the rubber. The other end of the elastic, suitably eyeletted, goes round the neck and then on to the nail end, holding the whole contraption hard against the strings and fingerboard.

It's cheap, a bit fiddly, and moderately effective, and, depending on the strength of the elastic, has a tendency to push an electric out of tune. Blueser Dave Kelly seems to use one with little problem, though. A friend in Liverpool got up to do a guest spot on a borrowed guitar and borrowed elastic capo, and totally blew his cred by somehow managing to catch the elastic with his thumb mid-number, unhooking it and sending it flying out into the audience, narrowly missing a front row punter. I have tried to duplicate this feat, regrettably without success, as it may have an effective anti-heckler application.

A fascinating variation on the elasticated theme is the Scott Tuning Capo. Instead of the central rubber, it uses six tongued and grooved pieces of plastic, offset on the central rod, and pushed together by a small spring. They can be separated and turned around individually to leave some string or strings un-capo'd while others are stopped. This will give instant open tunings, drones doubles and so on, but still leaves a normal tuning for flying up the neck with the bog-standard hot licks.

Currently it is available in flat fingerboard style, but a cambered version is on the stocks. It is fun, and as a bonus substitutes nicely for worry beads. It isn't yet widely distributed, so if you can't find it locally, call Ian Scott on (Contact Details). You'll find him very helpful if you have a specific size or strength need. Expect to pay about a fiver.

The Jim Dunlop capo ruled the roost for a long while — simple, light and unobtrusive. The rubber padded flat sits on the hoard and strings, and a piece of webbing attached to one end is taken round the neck to a lever, which slots into a choice of points on the top of the flat. The elasticity needed to pull the rig through its critical point is supplied by the flexibility in the legs at the webbing end of the lever. The 'Pro' version features enough adjustment via a buckle attachment to allow the user to set up to suit a particular guitar. Albert Lee has used one on a very light strung Tele with little trouble, but I've found it can affect tuning badly on a high fret bead. The Dunlop is available in flat or curved format, and is also now marketed under the Vision brand name by Rosetti. I hear a number of flamencos have used it in preference to cejillas because of its lightness.

Terry Gould has produced a variation on the Dunlop, which has a twisted contact area so that it can aim one way to suit a curved board, and the other way round to suit a flat board. It has the same inflexibility as the Dunlop for light strings on a high fret bead, but may be more difficult to use on a flat board. That way round, the lever is pointing downwards and can be knocked out more easily by the left hand.

Another Terry Gould tie-on capo put the leverage and hook at the back of the neck, the flat being a reasonably flexible hunk of rubber, grooved and held by a broken metal rod frame. It was actually not bad at coping with various cambers, but was fiddly to adjust for high or low on the neck, and affected tuning on light gauges. I have only one of several left, the others having been hurled into the darker comer of various stages after ineffective fumblings trying to put them on — three-handed job in my experience.

Frame capos date back a long way, to the beginning of the neurotic singer/songwriter folk scene, and many of them have the endearing crankiness of those times. The recent(ish) Picker's Pal is sublime, drawing on the traditional U shaped frame with a laterally opening bar. This bar is square, and on it fits a four sided rubber which will suit different neck thicknesses and different board cambers. Hanging loosely round the U frame, like descending knickers, is a bar holding the neck pad, which is forced against the back of the neck by a lever which ends in three cams, again to cope with different thicknesses of neck. It looks like something out of a particularly spiteful phase of the Inquisition, like its predecessor, the Hamilton.

The Hamilton, a U-shaped and swing bar American monstrosity, doesn't seem to be around the UK any more; at least, the calls I made looking for one were greeted by barely disguised sniggers. The bar was flat, with a piece of clear plastic tubing over it. If you had a curved fingerboard, then you simply jammed matchsticks in either end of the tube to approximate your camber. The pad is pushed against the back of the neck by a powerful spring, compressed for sliding the beast around by squeezing a huge lever on the side. This arrangement could give you a grip like Charles Atlas, but stuck out like some particularly vulgar bumper trim on an ancient Buick. The really extraordinary think about it is that it is being copied, by a few Japs, and by Wittner in Germany. The Wittner has a curved bar.

The Hamilton, and its copies, don't grip thin necks, the design dating from a time when men were men and necks were unplayable. But the capo that accompanied more UK soul-baring sessions was the B&M Rolif. The name is abbreviated loosely from Roland Spencer-Ford, a long retired director of Barnes and Mullins Manufacturing, and it appeared on a few other products including the nickel finger-picks. B&M tooled up for the Rolif capo in 1965 in Oswestry, and sadly this old-timer was discontinued very recently, though JHS market a Taiwanese copy. It was a U-shape with swing bar, rather neater than the Hamilton, and the squeeze on the neck was obtained from a screw up pad and guide pin (to prevent twist) operated from the bottom of the U. The flat bar still required the use of judiciously inserted matchsticks to achieve compatibility with a cambered board, but the finer adjustment available on the screw system did make it possible to stay reasonably in tune.

Perhaps the best frame capo for light string tuning stability is the Heriba. It's a rather ugly tough plastic device, with an upward opening gate bar, the string contact surface of which is formed as a row of small teeth encased in a clear rubber shield. It closes and holds by means of a ratchet arrangement at the side, and the teeth settle on and around the strings, pushing them down gently and firmly. Very easy to use, it can be put on one-handed, though the ratchet can make an alarming scrunching noise on an electro-acoustic.

The Wilkerson demonstrates an elegant simplicity of approach. An L shaped frame contains a rather too hard rubber insert to push against the strings, and a side lever pushes in to raise a three leaf spring against the back of the neck. This spring is the snag: it offers very little variation in possible pressure, and has a nasty tendency to split the clear plastic sheath.

Without a doubt, today's hotshot is the Californian Shubb. It's expensive — £12.95 at Andy's, £11.50 at Mairants — but very good, the best I've used so far. There are four configurations for flat classical, wide cambered (12-string), narrow cambered and banjo fingerboards. I've straightened out a narrow one for use on a flat electric fingerboard. The main frame is a gently curved V, the longer side of which has a rubber bar pad with a similar give and consistency to string callouses. The shorter side has a curved swing arm riveted on near the curve with a neck pad, and this is jammed against the back of the neck by a further swing bar at the end of the shorter side. Tension is held by a screw through this bottom swing bar, which is simply forced past a critical point on the curved arm.

It is inspired engineering of the 'why-didn't-I-think-of-that' variety. The only mild flaw is that it is possible to do the screw up too far so that forcing it through the critical point breaks or bends it, I have heard of one ham-fisted wally who has managed this, but have found it very hard to do myself. It's great with light strings, having plenty of fine adjustment, blindingly easy to use once correctly adjusted, and replacement bar rubbers are available should you do one in, though French chalk will delay wear. I bought three just before a tour in case I lost one or two in the on-road chaos. I needn't have worried, I lost a guitar stand instead.