Sometimes it also has a line under it. 1. To distinguish the 1 and the 7, the seven gets a cross bar. -7-

Depending on what your work context is, you may be instructed to always write the 1 and 7 that way, to avoid confusion. You may also be directed to write a the zero with a diagonal slash. ∅ to avoid confusion with a letter O. If you do a lot of inventory work or something, it becomes useful.