Set of ink cartridges, ink cartridge and ink jet printer

Ink cartridges (10a) to (10d) have different patterns from each other in positional relationship between projections (12a) to (12d) and insertion parts (16a) to (16d), respectively, and cartridge holders (20a) to (20d) have different patterns from each other in positional relationship between slits (22a) to (22d) and ink introducing tubes (21a) to (21d), as seen from the direction on which the ink cartridges (10a) to (10d) are mounted on the cartridge holders (20a) to (20d), respectively. At the same time, the respective patterns of the positional relationships in the ink cartridges and the cartridge holders coincide with each other per each of the four colors of black, cyan, yellow and magenta. Thus, any one of ink cartridges is prevented from inadvertently joined to any one of the cartridge holders to which the ink cartridge does not correspond to.