An asset's basis becomes very important when the owner sells the asset. The difference between the sale price and the basis is called a capital gain (if the sale price is higher than the cost basis) or a capital loss (if the sale price is lower than the basis). Capital gains are generally only taxable when the investor actually sells the asset. Realized losses can often offset these gains and thus lower the investor's potential capital-gains taxes. The length of time the asset is held, among other things, determines the tax effect of the gain or loss. Changes in tax rates also may influence an investor's concern about basis.
An asset's basis is usually based on its original purchase price, but sometimes people inherit assets rather than purchase them. In these cases, the basis of the asset becomes the value of the asset at the time the investor inherits it (this is called a step-up in basis).
Often, investors accumulate shares of the same stock at different prices over time. Because of this, when the investor sells some of the shares, he or she must identify which shares from the "inventory" were sold in order to calculate capital gains or losses. In general, investors want to minimize taxable gains by selling the shares with the highest basis first. However, if the investor cannot identify which shares are which, the IRS requires use of the first-in-first-out (FIFO) method, meaning that the investor must assume he or she first sells the shares that are held the longest. These older shares may not have the highest basis of the investor's inventory of shares, and thus the method could inflate the investor's tax bill.