[…] The difference between alternants of a unit may be partial or complete. It may occur in all units which have a stated feature (e.g. a given phoneme in a certain position), or in some units having a stated feature in common, or in a unique unit (or in several units which have no stated feature in common). The range of environments which determine the appearance of the alternant in question may consist of all morphemes which have a stated feature, or of only some of the morphemes having that feature, or of a unique morpheme (or of several morphemes having no common feature). […] If the difference between alternants of a unit is complete, it necessarily applies only to one unit. If the difference is partial, it may occur in one, some, or all units which have a stated feature. […] E.g. the complete difference between 'go' and 'wen-' exists only between two sequences of phonemes, hence (barring homonyms) only in this particular unit. However, the partial difference between 'knife' and 'knive-' can occur between any two sequences of phonemes that contain /f/ and /v/.
- Harris (1942a), a pag.177, n.21