Ai is the twelfth letter of the Ranjana varnamala, the second of the compound vowels, and the open counterpart of e. The grammatical analysis derives it in the same way, as the result of a combining with a vowel of the palatal group, but from the longer forms, which is why the tradition ranks it as the fuller or more open of the pair. In the classical pronunciation it is a long vowel of the front of the mouth, more open than e, and like e it has no short partner, so it stands alone in the series rather than in a pair. Its mark is written above the head stroke, in the same register as the mark for e and distinguished from it by shape, and the independent letter shown here appears where the vowel opens a word.
The pairing of e with ai and of o with au is a real structural fact rather than a mnemonic. It is why the last four letters of the vowel series run in that order, and why a reader who has learnt to distinguish the two marks above the line has learnt half the compound vowels at once. There is a further reason the compound vowels are grouped at the end rather than slotted among the simple ones. Sanskrit sound change works by combining vowels at the joins between words, and the compounds are what those combinations produce, so listing them separately keeps the derived apart from the given. A chart that mixed them would obscure the rule, and the rule is the point of the chart. Ranjana takes the arrangement over unchanged, as it takes the rest of the order, and adds nothing to it but its own hand.