[…] it is not necessary to set up more phonemes than the greatest number of different segments in any single environment (the greatest number of checks in any single column of the chart). If every environment had the same number of segments which contrasted in it […], each phoneme would consist of one segment from each environment, and the number of different phonemes would be the number of contrasting segments in any environment. However, we will usually find that in some environments there is a greater number of contrasting segments than in others. E.g., if we consider the small portion shown here of the English chart, we find [pʰ, tʰ, kʰ, b, d, g] in [#—V], against which we can match only [p, t, k] in [s—V]. If then we were to associate segments [p, t, k] with [pʰ, tʰ, kʰ] respectively, there would be no segments in the [s—V] environment to associate with segments [b, d, g]. The phonemes in which [b, d, g] are included would thus have no representative in the [s—V] position; they would be said not to occur there.
- Harris (1951), a pag.64