Ii is the long palatal vowel and the fourth letter of the Ranjana varnamala, standing to I as Aa stands to A. The mouth does the same thing in both cases, the front of the tongue raised and the lips drawn back, and the difference is duration. In the grammatical tradition that Sanskrit inherited, this pairing is stated as a rule rather than left as a fact about individual words: each of the simple vowels has a short and a long member, and the two behave as one sound for the purposes of the alternations that build the language, differing only in weight. That is why the varnamala teaches them adjacently and why a chart of the script sets them side by side. In writing, the long ii is marked in the same position as the short i, before the consonant it follows, but with a distinct stroke, so the two are told apart by shape rather than by placement.
The independent letter shown here is used only at the head of a word or where no consonant precedes. Because Sanskrit builds a great many of its feminine and derived forms with a final long ii, the mark is among the more frequent on a page of scripture, and a Newar scribe copying Sanskrit would have drawn it a great many times in a day. It is also one of the marks that shows how much of a Brahmic script's difficulty is a matter of small differences carefully kept. The two palatal vowels sit next to each other on the chart, occupy the same position beside the consonant, and are told apart by a detail of stroke, so a script that loses its precision in the small marks loses the distinction between two different words rather than merely looking untidy.