North-south (also called 12-6 or reverse scarf hold) is a top position rotated 180 degrees from side control: your head is near your opponent's hips and yours is near their head. Chest-to-chest pressure and underhooks pin them to the mat.
It is often used as a transitional position while rotating from one side to the other, and as an attacking position for the north-south choke — a tight pressing choke applied by dropping your chest onto the opponent's neck.
North-south is less common than side control as a stable pinning position but is useful for breaking grips, switching sides, and hunting for submissions when the opponent bridges into an awkward angle.