[Gretl-users] Periods in forecast variance decomposition

Hi, everyone. After estimating a VAR, Gretl offers a forecast variance decomposition under the "analysis" menu. The dialogue box requires the user to select at least two periods for the forecast variance decomposition, which makes me wonder why two periods are required ... if I select two periods, is the first one the last "in sample" period and the second one the first (and only) forecast period? Or, if not, what do the two periods represent? Thanks, Tim Cox