When an asset in respect of which capital allowances have been claimed is disposed of, there is a comparison between the disposal proceeds and the tax written down value. In the case of plant and machinery, this is generally done by reference to the entire pool of qualifying assets rather than on an asset-by-asset basis. If the proceeds exceed the tax written down value, a balancing charge arises which is added to trading profits. This has the effect of clawing back excess allowances previously claimed. For further discussion, see Practice note, Capital allowances: availability and types of plant and machinery allowances: Calculating WDAs, balancing allowances and balancing charges.