Drum unit, cartridge and coupling member

A cartridge comprising:

• a casing;
• a photosensitive drum supported by the casing such that the photosensitive drum is rotatable relative to the casing;
• and
• a coupling positioned at an end of the photosensitive drum, the coupling being operatively connected to the photosensitive drum such that the coupling is capable of transmitting a driving force to the photosensitive drum by rotating about a rotational axis of the coupling, the coupling including (i) a cylindrical main body positioned coaxially with the photosensitive drum and open to outside of the cartridge and (ii) a projection positioned in the cylindrical main body,
• wherein the projection is movable relative to the cylindrical main body between a first position and a second position such that an end of the projection is positioned closer to the rotational axis of the coupling when the projection is in the first position than when the projection is in the second position,
• wherein a part of the cylindrical main body is positioned outside of the photosensitive drum, and a part of the cylindrical main body is positioned inside of the photosensitive drum, and
• wherein the projection is at least partially positioned inside of the photosensitive drum.