 Complementary distribution is a concept that is associated with the physical view of the phoneme, which is the head term of a family of speech sounds, and whose members are phonetically similar and stand in complementary distribution, that is, their positions complement each other. In present-day English, the three members of the L phoneme are all alveolar lateral consonants. The slightly palatalized variant stands before vowels as in lip. The velarized L occurs after vowels as in pill, and the devoiced variant can be found after bilabial or velaplosives as in clip. So all family members have their own position, their distribution is complementary. Or take this German phoneme where one member follows front vowels as in ich, and one stands after back vowels as in buch, again a clear case of complementary distribution. The concept where the positions of the members of a family complement each other is not confined to phonology. Here is an example from literature. The split character in the novel The Strange Case of Dr. Jekyll and Mr. Hyde by Robert Louis Stevenson, where Jekyll is a well-respected scientist, and Mr. Hyde his evil alter ego, where one appears you would not expect the other. The two characters are in complementary distribution, aren't they?