A sample line of the French defence that shows that undefended pieces can be very vulnerable to tactics!1.e4e62.d4d53.Nc3dxe44.Nxe4Be75.Nf3Nf66.Bd3O-O7.Qe2b6Keep an eye on undefended pieces!8.Nxf6+Bxf6Black's only undefended piece is the rook on a8, and black loses because of that. The pieces that are attacked once and defended once, can be also considered as undefended, as the pawn on h7 is in this case.9.Qe4And to avoid checkmate, black has to give up the rook on a8, giving white a winning position.