This contribution introduces backchannel relevance spaces – intervals where it is relevant fora listener in a conversation to produce a backchannel. By annotating and comparing actualvisual and vocal backchannels with potential backchannels established using a group of subjectsacting as third-party listeners, we show (i) that visual only backchannels represent a substantialproportion of all backchannels; and (ii) that there are more opportunities for backchannels(i.e. potential backchannels or backchannel relevance spaces) than there are actualvocal and visual backchannels. These findings indicate that backchannel relevance spacesenable more accurate acoustic, prosodic, lexical (et cetera) descriptions of backchannel invitingcues than descriptions based on the context of actual vocal backchannels only.