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.
- Harris (1951), a pag.64

We say that any two distinct (non-equivalent) segments which have at least one environment in common, i.e. which are not compelementary throughout all their environments, contrast. More exactly, any two distinct segments which occur in the same environment (in the same column) contrast in that environment. They do not contrast in another environment in which only one (or neither) of them occurs.
- Harris (1951), a pag.75, n.32

In many languages we will find […] that one phoneme or another does not occur in particular environments in which other phonemes do, even when those phonemes are in general similar to it in distribution. This results from the fact that in some cases we may be unable to group segments into phonemes in a way that would satisfy the criteria of 7.4, because there are too many or too few distinct segments recognized in a given environment, or because two segments which we would like to group together happen to contrast.
- Harris (1951), a pag.90