I say, with God's guidance: The ankle that must be washed in ablution is the protruding bone that the shin bone ends at, according to the correct opinion; because matching the plural with the plural implies dividing individuals among individuals, like their saying: 'They rode their animals,' meaning each one of them rode his animal. Matching the plural with the dual does not imply that, like their saying: 'They wore two garments,' meaning each one of them wore two garments. So, God's choice of the plural in the ablution limbs, such as faces, heads, hands, and elbows, is intended to match one with one according to the rule. He chose the dual form for the ankle, so the dual matches each individual of the group, indicating that each leg has two ankles, which are the protruding bones, not the strap joint — the strap: the sandal's strap — as it is one in each leg. Refer to: Al-Sa'ayah p.71, and the margin of Issam Al-Din p.7/A, and God knows best.