Morphophonological analysis often involves an attempt to give a series of formal rules or constraints that successfully predict the regular sound changes occurring in the morphemes of a given language. Such a series of rules converts a theoretical underlying representation into a surface form that is actually heard. The units of which the underlying representations of morphemes are composed are sometimes called morphophonemes. The surface form produced by the morphophonological rules may consist of phonemes (which are then subject to ordinary phonological rules to produce speech sounds or "phones"), or else the morphophonological analysis may bypass the phoneme stage and produce the phones itself.