Rule of three

The rule of three belongs to frontal defense in rook and pawn versus rook. The defending rook stands in front of the pawn, toward the last rank where that pawn could become another piece, and checks the attacking king from that direction so the king cannot advance safely.

The guideline requires at least three ranks, or horizontal rows, between the pawn and the defending rook. This distance lets the rook repeat checks before the king can approach and attack it. As the pawn advances and the space shrinks, the defense may stop working.

The rule gives good drawing chances rather than a guarantee. The pawn's file, the kings' placement, whose turn it is, and any rook exchange leading to king and pawn versus king still affect the result. The pawns that began in front of the bishops and the two central pawns can produce exceptions even when the nominal distance is present.

It should not be confused with the rule of five. The rule of three measures room for frontal checks. The rule of five combines the pawn's progress with the number of files separating the defending king.