 Allophones of a phoneme that may appear in the same context without causing a change in meaning are called free variants. For example in English final plosives as in top step or lap may be aspirated as in top lap and uninspirated as in top or lap. They occur in the same context but there is no difference in meaning between top and top. We therefore cannot assign them to different phonemes. They're both allophones of the same phoneme and are referred to as free variants. Or take the realization of the initial sound in words beginning with H where the vowel U follows as in human or huge. You can pronounce them using the palatal approximate Y as in human or huge. Or you can take the voice palatal fricative Y as in human and huge. Both variants even share the same symbol. Some speakers make it even voiceless like in German hear me and say human and huge. So we have three free variants. Phonemically however there's no difference and the meaning doesn't change. Another example can be found in British English where the R between two vowels can be realized in two ways. We normally use the alveolar approximate as in carry or borrow but in a somewhat posh way of speaking English we can also use the alveolar flap as in carry or borrow. Thus as the American linguist William Laboff pointed out three variants may also have sociological significance.