A counter comprising: first ring member having first indicia and a second ring member having second indicia, each of said first and second ring members being rotatable in increments about a common axis, one or both of said first and second indicia indicating a count； a drive mechanism for rotating said first ring member； and a coupling mechanism for releasably coupling said first ring member to said second ring member, to allow said first and second ring members to rotate cooperatively when coupled and to allow independent rotating of said first ring member when not coupled； wherein said coupling mechanism comprises first and second engagement means, wherein said first engagement means comprise an arm having a slot and a contact end, and said first engagement means are movable radially outwardly and radially inwardly relative to said axis.一種計數器，其包含：一具有第一指標的第一環構件，及一具有第二指標的第二環構件，第一及第二環構件的每一者為可旋轉的而環繞一共同軸線增量，第一及第二指標的一或是兩者指示一計數；用於旋轉該第一環構件的一驅動機構；及一耦合機構，用於可釋放地耦合該第一環構件至該第二環構件，當耦合時，允許該第一及第二環構件配合地旋轉，及當脫離時，允許該第一環構件的獨立旋轉；其中該耦合機構包含第一及第二接合手段，其中該第一接合手段包含一臂，其具有一槽及一接觸端部，該第一接合手段為可相對於該軸線徑向向外地及徑向向內地移動。
