Tautology and pleonasm

These two terms are similar in meaning as both refer to unnecessary words. Some critics regard them as interchangeable but there is a useful difference.

Tautology refers to the repetition of words with the same meaning: big, huge, large man
Pleonasm occurs when the idea contained in one word is repeated by another: huge giant.
Pleonasm could be seen as a sub-type of tautology. The adjectives are: tautologous and pleonastic
Repetition can be used effectively for emphasis.

Principal and principle

These two words sound alike and so it is only in writing that their endings are confused.
Principal means head or chief and you can see the a in both. You could also think that the Principal of your school will never be your pal.
Principle means a belief and a way of remembering it is to think of asking someone to follow it please where you can see the ple in both.