Backup

Back-up roles are typically linked with a primary role, and don't get to use their action until an/the active role of the same name dies. Back-up Cop and Back-up Doctor (sometimes called Deputy and Nurse respectively) are common roles to utilise this modifier, but Trackers, Watchers and Roleblockers (amongst others) are all roles that occasionally might have a Back-up.

Most moderators design games with Back-up roles to reduce swing. Having a Cop and a Back-up Cop is a simple way to ensure a good chance of that investigation power to be successful, as opposed to a game with just a Cop, which is subject to a Day 1 lynch or Night 1 kill, completely eliminating that ability.

Some mods have also designed games with a Back-up existing without the primary role present, which typically serves to sew seeds of misdirection about the setup for the individual player and sometimes the town if that player dies. A Back-up in a game without a primary role present is essentially a Named Townie, which slightly differs to a Vanilla Townie for claiming purposes, even though they both have no abilities. Some players have expressed distaste for this facet of game design (this thread showcases a good cross-section of views on the topic).