Three letter rule

In English, the three letter rule (or short word rule) is the observation that content words are three letters long at least.

One- and two-letter words are mostly function words. Examples: I, at, he, if, of, or, etc. Therefore, content words almost all have at least three letters. Notice that content words with fewer than three phonemes get an extra letter or letters which are "phonetically redundant": the extra letters do not change the sound. Examples are such as ebb, add, egg, inn, bee, awe, buy, owe, etc.