Written down value rules govern depreciation adjustments and block asset accounting, determining taxable depreciation and adjustments on disposals. The definition of written down value sets taxable asset values: for assets acquired in the previous year it is actual cost; for earlier acquisitions it is actual cost less depreciation actually allowed (with a proviso limiting inclusion of certain earlier allowances for specified property). For a block of assets the written down value aggregates opening block values and is adjusted for additions, reductions on disposal (including scrap value, not exceeding the increased value), slump sale decreases limited by prior and hypothetical depreciation, and for later years by reducing opening block value by depreciation actually allowed and further adjustments including specified goodwill reductions, subject to the ceiling that reductions do not exceed written down value.