What is the difference between straight line and written down value depreciation?

In the straight line method (SLM) the depreciation amount is fixed each year because it is calculated on the original cost. In the written down value method (WDV), the rate is applied on the reducing book value each year, so the depreciation amount is highest in the first year and keeps falling. For example, at 10 percent WDV on 1,00,000 the first year is 10,000, but the second year is 10 percent of 90,000 = 9,000. Under SLM the asset can reach zero, while under WDV the value keeps reducing but theoretically never becomes zero. WDV is often preferred for assets like machinery that need more repairs as they age, because higher early depreciation plus lower later depreciation balances against rising repair costs, giving a more even total charge over the asset's life.