What does 3 AG mean on a fuse?

3AG is a code for the type of fuse. The "3" means the fuse is 1 1/4" (6.35 mm) in length and 1/4" (31.8 mm) in diameter. The "A" means it has no time delay when overloaded and the "G" means it has a glass construction. There is more information stamped on the fuse cap indicating the maximum amperes in fractions or whole numbers and the maximum voltage the fuse can tolerate.