I, and the Rule That Turns It into a Consonant

The most consequential thing about this vowel in Sanskrit is that it does not stay a vowel. When a dissimilar vowel follows it directly, i becomes the semivowel ya, and the whole sequence is rewritten. The grammarians state that once and let it apply everywhere, which is why the sandhi rules of the language are short enough to memorise: instead of listing the outcomes of every vowel meeting every other vowel, they name the semivowel each vowel answers to and give one rule of substitution. The same rule turns u into va and the syllabic r into ra, and the pairing of vowel with semivowel that the rule depends on is written into the order of the letters themselves.

The vowel is also the marker of a large class of nominal stems and the vehicle of the two strengthening grades. Raised one step it becomes e and raised a second it becomes ai, and a great many Sanskrit words are the same root at different points on that ladder. A reader who knows the ladder can often see the relation between two words that look unlike on the page, which is precisely what the tradition built it to do. In the Prakrit of the Ashokan edicts the vowel is if anything more frequent than in Sanskrit, because several older sounds fell together into it as the language changed.