Null morpheme
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A null morpheme is a morpheme that is realized by a phonologically null affix (an empty string of phonological segments). In simpler terms, a null morpheme is an "invisible" affix. It's also called zero morpheme; the process of adding a null morpheme is called null affixation, null derivation or zero derivation.
The null morpheme is represented as the figure zero (0) or the "empty set" symbol <math>\empty<math>.
The existence of a null morpheme in a word can be theorized by contrast with other forms of the same word showing alternate morphemes. For example, the singular number of English nouns is shown by a null morpheme that contrasts with the plural morpheme -s.
- "cat" = cat + -0 = ROOT("cat") + SINGULAR
- "cats" = cat + -s = ROOT("cat") + PLURAL
Also, a null morpheme marks the present tense of verbs in all forms but the third person singular:
- "(I) run" = run + -0 = ROOT("run") + PRESENT:Non-3rd-SING
- "(He) runs" = run + -s = ROOT("run") + PRESENT:3rd-SING
According to some linguists' view, it's also a null morpheme that turns some English adjectives into verbs of the kind of to clean, to slow, to warm. Null derivation, also known as conversion if the word class changes, is very common in English.
In languages that show the above distinctions, it's quite common to employ null affixation to (not) mark singular number, present tense and third persons (English is unusual in its marking of the third person singular with a non-zero morpheme, by contrast with a null morpheme for others). It's also frequent to find null affixation for the least-marked cases (the nominative in nominative-accusative languages, and the absolutive in ergative-absolutive languages).
A basic radical element plus a null morpheme is not the same as an uninflected word, though usage may make those equal in practice. In analytic languages with little or no derivational morphology, such as Mandarin Chinese, it makes no sense to speak of null morphemes or zero derivation.