A fascinating study by a great medieval historian (The Three Orders, The Age of the Cathedrals)--though it doesn't quite substantiate that subtitle: the patterns Duby finds in sexual behavior among 11th and 12th-century northern French aristocrats hardly seem to qualify as the matrix of 20th-century marriage. Still, their strategic importance is undeniable. Around the year 1000 matrimony was in a parlous state: polygamous noblemen freely repudiated old wives or acquired new ones to get male heirs, strengthen dynastic holdings, and satisfy their lust. Abduction and concubinage were rampant. Primogeniture condemned many younger sons to celibacy--if not necessarily to chastity. For its part, the Church looked down on marriage as a carnal last resort (which didn't prevent some priests from resorting to it) and generally relegated it to the secular sphere. On the other hand, the Church laid down an impossibly broad prohibition on incestuous unions, up to the seventh degree of kinship (which, since the nobility were all more or less interrelated, enabled restless husbands to ditch unwanted mates by suddenly ""discovering"" that their marriages had been invalid all along). But by the 1200s things had changed: the Church asserted itself by raising monogamy to a sacrament, assaulting some of the worst abuses, and demanding that its clerics be celibate. Laymen by and large went along with these reforms, because they spelled out and solidified the authority of men over women and heads of families over their members. ""Power could be preserved and extended only through its concentration,"" and so the Church crystallized into hard-and-fast principles while the knightly class crystallized into hierarchically structured families with the right to discipline and exploit the peasantry. In the meantime the lot of younger sons was improved by rising prosperity, the custom of parage (allowing them to hold in fief from their eldest brother enough property to set up a household), and the idealized cult of courtly love. Prof. Duby guides the reader through this tangled territory with immense learning and a relaxed, unpedantic style. He squeezes life out of monastic cartularies and other dry documents, but he wisely refrains from psychologizing over these highly conventional and impersonal sources. A vivid, graceful piece of scholarship.