The present invention provides a counter (203) comprising:a first ring member (201) having first indicia and a second ring member (202) 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 (205) for rotating said first ring member； anda 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.
