if you sell 100 shares at a loss today. and buy 100 shares of the same stock tomorrow. the wash sale rules apply. the loss will not be allowed for tax purposes. the buy can be in the same account, a different a/c of yours or your spouse's, or even an IRA. where you or your spouse are the primary beneficiaries. the loss is added to the tax basis of the shares you bought. if you buy 200 shares only 100 shares will be subject to the wash sale rules. if the purchase is done in the same account as the sale the broker should adjust the reported gain/loss on the year-end 1099-B. to reflect the wash sale. if a different a/c or IRA you'll have to make a manual adjustment.
say on day 1 you buy 100 shares of x
the next day you but another 100 shares of x
the next day you sell 100 shares of x at a loss using FIFO
the 2nd purchase causes the wash sale
using lifo the first purchase would cause a wash sale
