Principal variation

The principal variation, abbreviated PV, is the sequence of moves a chess engine considers best during its current analysis. A chess engine is a program that calculates positions and compares continuations. The first move in the PV is its preferred choice for the side to move; the following moves represent the replies it also considers best within the same search.

A PV should be read as the best line the engine has found so far, not as a prediction of what human players will actually do. It does not mean every move in the sequence is forced either. When several reasonable replies exist, the line simply presents one continuation consistent with the engine's current evaluation.

The variation can change as the reported depth, an indicator of search progress, increases or the engine spends more resources on the position. Discovering a new defense or a different sequence of moves can alter both the first move and the rest of the PV. Some interfaces offer MultiPV, a feature that asks the engine to show several candidate lines at once; this provides alternatives but divides search effort among more moves.