Correct spelling


Correct spelling, explaination: we spell this word without doubling t, because adding the suffix doesn’t change the pronunciation. In English double consonant often follows a short vowel, because the pronunciation changes (for example: bitter and biter). In this case, therefore, it is correct to spell outer, not outter.

Definition of outer:
adjective, on the external part
Have you seen “Plan 9 from Outer Space”? It’s such a silly horror it is funny!

Incorrect spelling


Incorrect spelling, explaination: outter is incorrect, because additional t is unnecessary – it simply doesn’t change anything. By adding suffix -er we don’t change pronunciation of this word, so the correct form is outer. For example the word inner needs double consonant, because it follows the short vowel, and thus word iner would be pronounced the same way as liner. Out however has got long vowel before the consonant.