embodiment the strap 4 is held in the base 3 by a two position eccentric cam 34 that can pivot parallel to the axis 3 o 39 clock 9 o 39 clock between a position where the strap is wedged between the cam 34 and the base 3 and an adjustment position not illustrated where the interstice between the cam 34 and the base 3 is sufficient to have the strap 4 slide the strap is thus held by compression of an arbitrary portion of one of its upper or lower sides and or by friction against the elements 3 and 34 as previously mentioned the strap is preferably of an elastic material so that it takes up its form again when the pressure of the cam 34 is relaxed the cam 34 can preferably be freed only when the clasp 1 2 3 is open when the watch is closed the case 1 blocks the cam 39 s rotation thus preventing an accidental maladjustment of the strap 39 s length the cam 34 is preferably arranged in such a manner as not to be disengaged by pivoting without a certain resistance even when the clasp is open so as to allow the clasp to open without risk of accidentally misadjusting the strap 39 s length the resistance to the pivoting.