Guna refers to a set of normal-length vowels that are less reduced than the basic set (in modern terms, the zero grade ), but more reduced than the vṛddhi vowels (in modern terms, the lengthened grade ). As an example, ṛ, i, u are basic (zero-grade) vowels, with corresponding guṇa (full-grade) vowels ar, e, o and vṛddhi (lengthened-grade) vowels ār, ai, au . (This is more understandable once it is realized that, at an earlier stage of development, Sanskrit e and o were ai and au , and Sanskrit ai and au were āi and āu .) Guna corresponds to what is now termed the full grade in Indo-European ablaut . Another orthography and phonology concept related to Guna is Briddhi .