An allophone in phonology is one of multiple positional variants of a single phoneme. Allophones are conditioned by the phonetic environment and may be predictable on the basis of the phonological features of that environment. Unlike phonemes, allophones do not distinguish meaning and most native speakers are not aware of the automatic alternation between allophones.
In English, the [k] in keep has a secondary palatal articulation, caused by the high front vowel [i:] following it, something that is lacking in the [k] of cool. Most linguists also consider the German sounds [x] and [ç] allophones of the same underlying phoneme.