We speak of families as though we all knew what families are. We identify, as families, networks of people who live together over periods of time, who have ties of marriage and kinship to one another. The more one studies family dynamics, the more unclear one becomes as to the ways family dynamics compare and contrast with the dynamics of other groups not called families, let alone the ways families themselves differ. As with dynamics, so with structure (patterns, more stable and enduring than others): again, comparisons and generalizations must be very tentative.