:Conditioning factor:

Some aspect of the environment of a phoneme which determines which of its allophones will be used. Examples are: the stress status of the syllable, the position within the syllable, the following or preceding sound. For instance, the Castilian Spanish /b/ phoneme has two allophones: [b] and [β]. The conditioning factor governing the choice between them is what immediately precedes the phoneme. If this is a pause or [m], then the [b] allophone is used. Elsewhere the [β] is usedː saber (“to know”) = [saˈβer], but también (“also”) = [tamˈbjen].