A notation aid where the letter B denotes a backward shift of one period. Thus, B operating on Xt (noted as BXt) yields, by definition, Xt-1. Similarly BB or B2 is the same as shifting back by two periods. A first difference (Xt-Xt-1) for a time series can be denoted (1 B) Xt. A second-order difference is denoted (1 B)2 Xt . See differencing.