Within a century or two of its creation the Tibetan script had a task it was not shaped for: rendering Sanskrit exactly, in the names, mantras and technical terms of a translated Buddhist canon. Approximation would not do, because a mantra depends on its sounds and a proper name has to be recoverable. So a second apparatus was built beside the thirty letters.
Three devices carry most of it. The retroflex consonants of Sanskrit, which Tibetan does not have, are written as mirror images of the dental letters, so that a reversed ta writes the retroflex, a reversed na writes the retroflex nasal, and so on. The voiced aspirates, likewise absent from Tibetan, are written as the plain voiced letter with a subjoined ha, so that ga with ha beneath it writes the Sanskrit gha. Long vowels, which Tibetan does not distinguish, are written by subjoining the small a beneath the letter, or by doubling the vowel sign.
None of this entered the recitation. A Tibetan child still learns thirty letters and four vowels, and the reversed forms and the rest are taught afterwards as a specialist extension for a specialist purpose.
The result is a script that can hold two languages at different levels of precision, and a page of Tibetan Buddhist writing in which the ordinary text and the transliterated Sanskrit are visibly different in texture.