What is used on uniforms of some US military or paramilitary personnel to indicate number of years' service?

Question

What military slang term for members of the US Marine Corps was derived from a special collar once worn around the neck by both American and British Marines and soldiers from 1798?

Answer 1

Hash marks
The hash mark is an embroidered diagonal stripe on the lower part of the sleeve. Each mark indicates a period of duty, three years in the Army or four years for naval services. The US Air Force uses a ribbon (to mark four years service) instead of hash marks.