It is disputed how many vowels Proto-Indo-European has or even what counts as a "vowel" in the language. It is generally agreed that at least four vowel segments existed, which are typically denoted as and All of them are morphologically conditioned to varying extents. The two long vowels are less common than the short vowels, and their morphological conditioning is especially strong, suggesting that in an earlier stage there may not have been a length opposition, and a system with as few as two vowels (or even only one vowel, according to some researchers) may have existed.