You’ve now seen that the mental grammar of every language organizes speech sounds differently. A pair of sounds that contrast with each other in one language might be allophones of the same phoneme in another language. Even without knowing another language, you can now use the tools that linguists use to analyze phonological data from a language: you can look for minimal pairs to find evidence of phonemic contrast, or you can analyze environments and identify complementary distribution to find evidence of allophonic variation.
Many processes of allophonic variation in the world’s languages apply not just to pairs of segments, but to natural classes of sounds. The notation of feature matrices helps to identify the natural classes that undergo allophonic variation, and the natural classes of environments that lead to (or “condition”) this variation.