The Semivowels as a Block

The four semivowels sit between the stops and the sibilants in the order, and the position is not a leftover. The phoneticians classified them as sounds made with a narrowing of the vocal tract that does not amount to a closure, which puts them between the stops, where the tract closes completely, and the sibilants, where the narrowing produces friction. The order of the table is a gradient as well as a tour of the mouth.

Their pairing with the vowels is the more consequential idea. Each semivowel is treated as the consonantal counterpart of a vowel formed at the same place, and Sanskrit grammar makes constant use of the relationship: vowels become semivowels before other vowels, semivowels become vowels in certain formations, and the strengthening grades that build so much of the vocabulary move between the two. A description that did not connect them would have to state the same rules twice.

For a reader this is one of the places where knowing the table pays. A form that looks unfamiliar is very often a familiar root with a vowel turned into its semivowel or the reverse, and recognising the pairing is what makes the underlying word visible. The four letters are learned as a block because they behave as one.