Film leader

Film leader
Length of protective film at the beginning of a roll of unexposed or processed film.

Leader (Film Leader).
Part of film attached to camera take-up spool. 35 mm film usually has a leader of the shape originally designed for bottom-loading Leica cameras, although most cameras simply need a short taper.
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7 A film leader retriever or bottle opener to get at your film.
8 A timer. A watch will do but a proper timer is easier to keep track of.

In the earliest days, the photographer had to load the film into reusable cassettes and cut the film leader. In 1934, Kodak introduced a 135 daylight-loading single-use cassette.

For example, perhaps you want to change rolls of film in the middle - maybe going from colour to black and white - and then you want to reload the partially used film later. If the film leader is out then this is pretty easy to do.

See also: Film, Leader, Light, Back, Type