However, such a tripartite system is not found in any descendant language (Sanskrit had a fourfold distinction, including a voiceless aspirated series), and it is typologically rare across attested languages. The absence or rarity of *b (see below) is also unusual. Additionally, Proto-Indo-European roots have a constraint that forbids roots from mixing voiceless and voiced aspirate stops or from containing two voiced stops. All that has led some scholars to change the reconstruction by replacing the voiced stops by "glottalized" and the voiced aspirated stops by "plain" voiced. Direct evidence for glottalization is limited, but there is some indirect evidence, including Winter's law in Balto-Slavic as well as the parallel development of voiceless consonants and voiced aspirated consonants in Germanic: both became fricatives and glottalized (plain voiced in the earlier theory) consonants remained stops.