For nylon string guitar,
or any guitar with a perfectly flat fretboard.
2.25 inches, flat.
It is important to note that almost NO steel string guitars have a truly flat fretboard. Even if your steel string guitar has only a "very slight" curve, it still should use our S1 model.
But if you play classical, flamenco, or folk guitar with nylon or gut strings, then chances are that your fretboard IS perfectly flat and slightly wider, and the S2 is the capo for you.