So far we have seen bu only as a suffix to vowel cmavo to produce vowel lerfu words. Originally, this was the only use of bu. In developing the lerfu word system, however, it proved to be useful to allow bu to be attached to any word whatsoever, in order to allow arbitrary extensions of the basic lerfu word set.

Formally, bu may be attached to any single Lojban word. Compound cmavo do not count as words for this purpose. The special cmavo ba'e, za'e, zei, zo, zoi, la'o, lo'u, si, sa, su, and fa'o may not have bu attached, because they are interpreted before bu detection is done; in particular,

is needed when discussing bu in Lojban. It is also illegal to attach bu to itself, but more than one bu may be attached to a word; thus .abubu is legal, if ugly. (Its meaning is not defined, but it is presumably different from .abu.) It does not matter if the word is a cmavo, a cmene, or a brivla. All such words suffixed by bu are treated grammatically as if they were cmavo belonging to selma'o BY. However, if the word is a cmene it is always necessary to precede and follow it by a pause, because otherwise the cmene may absorb preceding or following words.

The ability to attach bu to words has been used primarily to make names for various logograms and other unusual characters. For example, the Lojban name for the “happy face” is .uibu, based on the attitudinal .ui that means “happiness”. Likewise, the “smiley face”, written “:-)” and used on computer networks to indicate humor, is called zo'obu The existence of these names does not mean that you should insert .uibu into running Lojban text to indicate that you are happy, or zo'obu when something is funny; instead, use the appropriate attitudinal directly.

Likewise, joibu represents the ampersand character, “&”, based on the cmavo joi meaning “mixed and”. Many more such lerfu words will probably be invented in future.

The . and , characters used in Lojbanic writing to represent pause and syllable break respectively have been assigned the lerfu words denpa bu (literally, “pause bu”) and slaka bu (literally, “syllable bu”). The written space is mandatory here, because denpa and slaka are normal gismu with normal stress: denpabu would be a fu'ivla (word borrowed from another language into Lojban) stressed denPAbu. No pause is required between denpa (or slaka) and bu, though.