"On defaultly-configured Unix systems, and on ancient dumb terminals and teletypes, the tradition has been for the TAB character to mean move to the right until the current column is a multiple of 8. This is also the default in the two most popular Unix editors, Emacs and vi.
In many Windows and Mac editors, the default interpretation is the same, except that multiples of 4 are used instead of multiples of 8."
[1] http://www.codinghorror.com/blog/archives/001254.html