In this vowel system, the "neutral" vowels ("i" and "u") were free to occur in a word with any other vowel or vowels. The lone front vowel ("e", but generally pronounced like Mandarin ) never occurred in a word with either of the regular back vowels ("o" and "a"), but because the rules of vowel harmony are not perceptible with diphthongs, the diphthong "eo" occurs in some words, i.e. "deo", "younger brother", "geo", "a mare", "jeo", "department", "leole", "to discuss", "leose", "building", and "šeole", "to embroider", "to collect".