The system of accounting in which the assets of firms are valued and their depreciation allowances are calculated using the costs of replacing their buildings and equipment. Where buildings and equipment can be replaced exactly, this might seem an ideal accounting method. However, particularly in the case of equipment, owing to technical progress, exact replacements are frequently not available, and would not be worth installing even if they were. In such cases calculating the cost of appropriate replacements for equipment is very much a matter of judgement.