La is the twenty-sixth of the thirty consonants, an alveolar lateral. Like ra it belongs to all three of the stacking systems: it is one of the three superscripts, one of the four subscripts, and one of the ten suffixes.
Written above a root letter as the la mgo it is silent in modern Lhasa speech and the syllable takes high tone in its place. Written below, it produces a value of its own. Written after, it closes the syllable.
The syllable la is also the commonest particle in written Tibetan. It marks the dative and the locative, it appears in a range of adverbial and verbal constructions, and it turns up in almost every sentence of any length. A reader therefore meets the letter far more often as a piece of grammar than as the head of a word, in the same way as with pa.
Because the particle is so frequent and so short, it is one of the first things a learner stops decoding and starts seeing. A page of Tibetan read at speed is parsed largely by its particles, and la is the one that appears most.
La is not among the five prefixes either, so like ra it takes every position in the syllable except the first.
As an initial it carries low tone in modern Lhasa speech. Its behaviour as a subscript is the least predictable of the four, because a root letter with a subjoined la is pronounced in a way that follows from neither of the two letters taken alone and simply has to be learnt with the words that contain it.