Another important factor in the evaluation of chess positions is the "pawn structure" (sometimes known as the "pawn skeleton"): the configuration of pawns on the chessboard. Since pawns are the least mobile of the pieces, the pawn structure is relatively static and largely determines the strategic nature of the position. Weaknesses in the pawn structure, such as isolated, doubled, or backward pawns and , once created, are often permanent. Care must therefore be taken to avoid these weaknesses unless they are compensated by another valuable asset (for example, by the possibility of developing an attack).