Correct spelling


Correct spelling, explanation: outer is the correct spelling because the base word out consists of two vowels and a consonant. In such a case, when adding a suffix, there is no need to double the last consonant. Therefore the word should be spelled outer, not outter.

Definition of outer:
adjective, external, situated towards the outside
The outer part of the device is made of waterproof material.
This product cleans outer surfaces very well.

Incorrect spelling


Incorrect spelling, explanation: outter is an incorrect form, and outer is correct because there is no consonant-vowel-consonant combination at the end of the base word out. In such a case, we double the last consonant, but it’s not the case with the word out. Therefore the correct form is outer – without doubled t.