Book value of equity consists of two economically different components: retained earnings and contributed capital. We predict that book-to-market strategies work because the retained earnings component of the book value of equity includes the accumulation and, hence, the averaging of past earnings. Retained earnings-to-market predicts the cross section of average returns in U.S. and international data and subsumes book-to-market. Contributed capital-to-market has no predictive power. We show that retained earnings-to-market, and, by extension, book-to-market, predicts returns because it is a good proxy for underlying earnings yield (Ball, 1978; Berk, 1995) and not because book value represents intrinsic value.