The instrument is composed of two concentric rings mounted on a straight arm with an attachment hole at its free end. The outer ring is engraved on one side with an undivided double 12 hour scale (I to XII, I to XII) engraved anti-clockwise so as to correspond correctly when the 24 touch pins set on the other side are numbered off in a clockwise direction. Beneath the touch pins, on the inner edge of the ring, is an unnumbered 360? scale. Within this ring and free to rotate is a second ring engraved on its circumference with a calendar scale reading to one day but without indication of leap year. In the space between the month of March and the boss at the centre of the rings is a subsidiary ring and a further fourth ring, of which only one half is open and which may be rotated within the subsidiary ring. A small notch is cut at the centre of the diameter of the fourth ring. Mounted at the centre of the instrument are two indexes, one with a flat pointed end, the other with a square end returned through 90?. Both these however may be replacements.
