AMERICAN kinship has been described most fully in those aspects in which it differs from other social institutions. The nuclear family stands out from the occupational world like a figure on a ground. The specific technical competence required in the occupational world contrasts sharply with the requirements of familial loyalty and devotion to parents, siblings, and children2 (Parsons 1943). Although American kinship is sharply differentiated from other social institutions, both empirical and theoretical considerations suggest significant points of continuity and consistency between kinship and other parts of the social system (Schneider and Homans 1955). We will describe here findings which draw attention to a feature of American kinship which shows this consistency with other features of the American social system and which has not yet been adequately described. This feature is a solidarity among siblings3 (Pherson 1954:199-202) which we believe to be a special case of the generational solidarity which is a feature of American life4 (Codere 1955). This is an age-graded society. We make a distinction between a nurseryschool child and a kindergarten child, and both differ from a school-aged child. We have pre-teens and teens and sub-debs and debutantes, and young married couples. At this point, we seem to relax our efforts, only to begin again with middle age, late middle age, older people, and finally old age.5 This rather fine age-grading probably has two important sources; it is generated by the equalitarian ideology which renders us less comfortable with vertical relationships than with horizontal ones, and second, by an industrialized society of refined technology in which it is important to keep the young out of many key roles until they know enough to fill them, and to remove the old before what they know is obsolete. We are familiar with the structural evidence of age-grading. For example, there are the adolescent gangs and cliques (Whyte 1955) and in the middle class, there are women's clubs. In working and lower class societies, there are important men's groupings. We will present here preliminary evidence that solidarity among siblings is the counterpart in the kinship structure of this tendency to horizontal solidarity, and we will trace out some functional implications of this feature.