Nga is the velar nasal and closes the first class. It never begins a Sanskrit word, and it occurs essentially only immediately before another velar, so the independent letter is written rarely and the sound is met almost entirely inside conjuncts. A learner encounters it first as part of a cluster and only afterwards as a letter with a place of its own.
Its place is nonetheless fixed, because each of the five classes ends with the nasal made at the class point of articulation, and that closing nasal is what turns the five classes from a list into a system. Nasals ordered by place are one of the most durable results of Indian phonetics, and they are the part of the analysis most obviously absent from the writing systems of the Mediterranean, which have one nasal letter for the front of the mouth and one for the back and no framework connecting them to anything.
Because the letter appears chiefly in clusters, it also belongs to the part of the script that is learned separately. A conjunct is not always the sum of its letters in Siddham, and the shapes that this nasal makes with the stops of its own class are among those a reader acquires as units.
Its scarcity as an independent letter has a practical effect on the evidence. A character written rarely is written carefully, so its form drifts less than that of a common letter, and a palaeographer looking for a stable point of comparison between hands will often find one among the letters nobody ever had cause to hurry.