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What are combining forms as opposed to prefixes and suffixes (general term affix)?
In English, many words have special combining forms which appear only in compounds or only in compounds and derivatives, e.g., electro-, the combining form of electric, appears in such compounds as electromagnet. A combining form can be distinguished from an affix (i.e., prefix or suffix) by one of the following: 1) its ability to occur as one immediate constituent of a form whose only other immediate constituent is an affix (as cephal- in cephalic), 2) its being an allomorph of a morpheme having another allomorph that may occur alone, or 3) is distinguished historically from an affix by the fact that it is borrowed from another language in which it is a combining form or a word. Another way to explain the three types of combining form is: 1) forms borrowed from Greek or Latin that are derivatives of independent nouns, adjectives, or verbs in those languages (for example, cardio-, -phile) and usually appear only in combination with other combining forms of Greek or Latin origin (for example, bibliophile, cardiology); 2) the compounding form of a free-standing English word where this type of combining form usually has only a single, restricted sense of the free word, and may differ from the word phonetically (for example, -land, -man, -proof, -wide); and 3) a form extracted from an existing free word and used as a bound form, usually keeping the meaning of the free word (for example, -aholic, heli-, mini-). In word formation, a combining form may conjoin with an independent word (mini- + skirt), another combining form (photo- + -graphy), or an affix (cephal- + -ic). An affix is different because it can be added to either a free word or a combining form but not solely to another affix.