Proper apostrophe usage? [duplicate]

Question

Possible Duplicate:
What is the proper way to write the plural of a single letter? (another apostrophe question)
Plurals of acronyms, letters, numbers — use an apostrophe or not?

Take for example, you ask somebody is there's two of one letter in somebody's name.

I normally write:

Two n's in Hannah?

Because this looks weird:

Two ns in Hannah?

Is there an English rule that approves of this apostrophe usage? Do you do it? Am I wrong for thinking ns looks wierd?

Answer 1

The British linguist David Crystal has written that the apostrophe ‘avoids an awkward juxtaposition of symbols, especially with plurals’. ns is one such case. Write it as n’s.