A costing system sometimes applied to production carried out by a series of chemical or operational stages or processes. Its characteristics are that costs are accumulated for the whole production process and that average unit costs of production are computed at each stage (see average costing). Special rules are applied in process costing to the valuation of work in progress, normal losses, and abnormal losses. In process costing it is usual to distinguish between the main product of the process, by-products, and joint products. Compare continuous-operation costing.