A characteristic feature of the Mongolian language is that of vowel harmony, a feature which is also common in Turkish and to a lesser extent in Manchurian as well. The concept of vowel harmony is extremely straightforward, and simply means that a word can only contain either back vowels (a, o, u) or front vowels (e, ö, ü), but not both at the same time, with the exception only of a certain limited amount of words, the majority of which are foreign. The vowel i is considered neutral and can therefore occur in both front and back voweled words, but when i occurs in all syllables the word is considered to be front voweled. Vowel harmony also affects two other sets of letters, γ/q and g/k, with the former occurring only in back-voweled words and the latter only in front-voweled words.
Examples of back vowel words
Back voweled word can only contain the back vowels a, o and u or the neutral vowel i, and cannot contain the consonants g and k (except when k is followed by the letter i).
Examples of front vowel words
Front voweled word can only contain the front vowels e, ö and ü or the neutral vowel i, and cannot contain the consonants γ and q (except in a small and limited number of words).
Examples of front and back-voweled words containing 'i'
Front neutral vowel i can occur alongside all vowels (back and front) and all consonants.
Examples with 'i' in all syllables
When i occurs in all syllables the word is regarded as front vocalic and any suffixes/case markers joined to the word must also be front vocalic.
Back vowel words with 'γ' and 'q'
Only back-voweled words can contain the consonants γ and q, with the exception of a limited number of words (see below)
Front vowel words with 'g' and 'k'
Only front-voweled words can contain the consonants g and k, with the exception of a limited number of words (see below) and when k is followed by the letter i.
Examples of words which violate vowel harmony
A very small number of nouns break from the rules of vowel harmony, these are mainly words of foreign origin.