|A few types of capo|
Like a passive third hand on your fretboard, the capotasto (to give it its full name, literally: the head of the touch) can be a simple device to raise the pitch of your instrument or a complex mechanism to contrive new tuning combinations. Gone (almost) are the days of the elastic strap and buckle systems that strangled the neck and threatened to take out an eye during an inadvertent explosive release. These days capos are engineering marvels, both simple and complex.
The main consideration is to have something closely approximating the force and form of a human finger on the strings which, in most cases, means applying pressure from around the neck. You might think that this is simple and straightforward, but the number of mechanisms developed for achieving this is startling: elastic straps, spring loading, cam mechanisms, lever-tensioning, friction locking and over-centre locking are just some of the engineering principles employed for this seemingly simple task.
Whatever its design, it's important that it applies only the necessary pressure. Too much and the tuning is compromised; too little and the strings 'buzz' on the leading fret. It should be possible to tune the instrument with the capo on (usually necessary as the intonation is inevitably compromised when capoing).
But it doesn't stop there. How about capos that only fret certain strings and leave others open (partial capoing). Or low-friction capos that are designed to be more or less permanently installed on the instrument that can be slid quickly into position for a rapid key change while playing.
Partial capoing is used on guitars, most commonly on the second fret across the A, D and G strings making the effective tuning of E,B,E,A,B,E. (Those familiar with DADGAD tuning will note that this is the same tuning ratio, 2nd fret). This gives the dubious benefit of being able to play in a faux open-tuning, but use conventional chord shapes. I've always thought this defeats the purpose of open tuning in the first place. It's not uncommon to see a player use a normal capo and a partial capo together.
The use of capos is not restricted to guitars. There are small ones for mandolins and ukes and the afore-mentioned low-friction, sliding type for use with open tuned instruments like the Irish bouzouki (or mandola, or cittern) that sit behind the nut when not in use and can be slid like a collar to the desired fret mid-tune.
Your choice of capo will depend on your style and mode of playing - many are designed to be positioned with one hand (important if you wear finger- and thumb-picks). Some are discreet and some are imposing. Another consideration (recently demonstrated to me) is that different capos make your instrument sound differently, so it's worth trying a few. That's the thing about capos. You can try them at the shop. Not like toothbrushes.