Lojban does not have punctuation marks as such: the denpa bu and the slaka bu are really a part of the alphabet. Other languages, however, use punctuation marks extensively. As yet, Lojban does not have any words for these punctuation marks, but a mechanism exists for devising them: the cmavo lau of selma'o LAU. lau must always be followed by a BY word; the interpretation of the BY word is changed from a lerfu to a punctuation mark. Typically, this BY word would be a cmevla or brivla with a bu suffix.

Why is lau necessary at all? Why not just use a bu-marked word and announce that it is always to be interpreted as a punctuation mark? Primarily to avoid ambiguity. The bu mechanism is extremely open-ended, and it is easy for Lojban users to make up bu words without bothering to explain what they mean. Using the lau cmavo flags at least the most important of such nonce lerfu words as having a special function: punctuation. (Exactly the same argument applies to the use of zai to signal an alphabet shift or ce'a to signal a font shift.)

Since different alphabets require different punctuation marks, the interpretation of a lau-marked lerfu word is affected by the current alphabet shift and the current font shift.