The present paper, dealing with nylons 66, 610, and 612, is part of a study on a series of polyamides as crystallized from solution. The lath-shaped crystallization products, aggregated in the form of sheaves, possess the characteristics of chain-folded single crystals. In particular, they can be collected in the form of oriented mats which display arced X-ray reflections both in the wide- and low-angle regions. This enables the fold surfaces to be indexed as 00l. In a given polymer the long spacing has a characteristic value and for different polymers it varies in the ratio of the monomer lengths. The measured layer thickness corresponds closely to four-unit cell lengths. The fact that this holds for all three polymers implies that the fold length is determined by the number of hydrogen bonds between the stems. In view of the small value of this figure (due to the large identity period) the long period can only be apportioned between straight stems and folds in a limited number of ways which enables some concrete suggestions to be made on the possible constitution of the fold itself. On annealing, the fold length increases preferentially by factors of 2. A mechanism with general implications for the refolding behavior of polymers is proposed.