The gap is the greatest distance at which, under certain given conditions, a priming cartridge is capable of initiating a receiving cartridge (receptor). The same explosive is usually used both as primer and receptor, although the gap distance in such a case will also be affected by any change in strength that may occur in the explosive. The gap test can be carried out with the cartridges unconfined or confined, for example, in tubes, in air, or in water. The test gives, for example, information about changes in the explosive due to aging, moisture, temperature, etc.