XXXIII. The apostrophe marks the omission of a letter or letters: as, His, I'm, don't, man's. (For the possessive case see sect. 73).

XXXIV. The apostrophe marks the omission of figures in dates.

The boys of '61. The class of '09.

XXXV. The apostrophe sometimes marks the plural of figures and letters.

Cross your t's and dot your i's. His 7's look like his 9's.