Floating tone

A floating tone is a morpheme or element of a morpheme that contains no consonants, no vowels, but only tone. It cannot be pronounced by itself, but affects the tones of neighboring morphemes.

An example occurs in Bambara. Bambara has two phonemic tones, "high" and "low." In this language, the definite article is a floating low tone. With a noun in isolation, it docks to the preceding vowel, turning a high tone into a falling tone: [bá] "river;" [bâ] "the river". When it occurs between two high tone, it downsteps the following tone:
*IPA| [bá tɛ́] "it's not a river"
*IPA| [bá tɛ̄] (or IPA| [bá ^↓ tɛ́] ) "it's not the river"

Also common are floating tones associated with a segmental morpheme such as an affix. For example, in Okphela, a Niger-Congo language of Nigeria, the main negative morpheme is distinguished from the present tense morpheme by tone; the present tense morpheme (á-) carries high tone, whereas the negative past morpheme (´a-) imposes a high tone on the syllable which precedes it:

*oh á-nga "he is climbing"
*óh a-nga "he didn't climb"

Floating tones derive historically from morphemes which assimilate to the point where only their tone remains.