1 The national flag which is flown from a jackstaff on the stem of naval ships when at anchor.
2 Pilot jack, originally the name given to the flag, which was a Union flag surrounded by a white border, flown by ships in need of a pilot. Later, this requirement was met by ‘G’ flag in the International Code of Signals, which has yellow and blue vertical stripes.
3 Cargo jack, sometimes also known as a jack screw, an appliance used in the holds of merchant ships, before the introduction of container ships, for moving cargo and for compressing cargo such as cotton, hides, etc. into as small a space as possible to increase the carrying capacity.