Nouns may be singular or plural. There are also three "forms" that indicate whether the noun is indefinite, definite or demonstrative. "Form" and number are indicated conjointly by an enclitic on the noun phrase. The singular definite enclitic is -ǒ or -ǎ. Some authors always write the ending with a rising tone mark even if it is not ambiguous and even if it is not truly a rising tone. The other endings are in the table below. The definite and the demonstrative endings replace any final vowel. See Hamani (1980) for a discussion on when to add -ǒ or -ǎ as well as other irregularities. See Tersis (1981) for a discussion of the complex changes in tone that may occur.