Jha is the voiced aspirate of the palatal class and one of the least used letters in the syllabary. It occurs in a small number of roots, most of them onomatopoeic or of uncertain origin, and it is rarer still in the Buddhist texts that were copied in Siddham. A scribe might write it a handful of times in a working life.
Its presence in the table follows from the same principle that puts the unused vowels there. The arrangement describes the sound system as the phoneticians analysed it, and a class of five missing its fourth member would break the pattern that makes the other four predictable. A learner who can recite one class can recite them all only because none of them has a hole in it.
The letter behaves like any other consonant sign on the page. It takes the vowel marks, carries the inherent short a, and can enter conjuncts, though its low frequency means very few of those combinations are ever needed. It stands fourth in its class, in the slot every class reserves for the letter carrying voicing and aspiration at once, and a reader can name it from that position without ever having met it in a word.
It is the sort of letter that separates two ways of describing a script. An inventory of what actually gets written would leave it out; an inventory of what the tradition transmits cannot. This atlas records the second, which is why the Siddham table here has forty-seven entries rather than the smaller number a count weighted by frequency would produce.