The following is an overview of the phonemes that are reconstructible for Middle Common Slavic. There are few reconstructible neuters; those that can be reconstructed were irregular. In Oceania, there are several cognate sets reconstructible for mountain apples and related species. A detailed system of allophony is reconstructible, known as consonant gradation. Stative roots were rare; perhaps the only reconstructible stative root verb was "know". It is possible that many reconstructible "a"-stem neuters originally belonged to this class, however. For Old Church Slavonic, the following segments are reconstructible. For example, the reconstructible common ancestor of the Indo-European language family is called "Proto-Indo-European".