For rugby union, the screw-in stud is preferred, especially in the positions of prop, hooker, and lock, where more grip is required for contested scrums. These screw-in studs have to be of a maximum length of 21 mm. These boots are often heavier than appropriate for other types of football. One of the more obvious differences between association football and rugby boots is the formation of the studs - rugby boots typically have no fewer than nine studs whilst those worn for soccer can have a minimum of six. Also, some rugby boots tend to have a high cut around the ankles but this type is becoming less popular, especially at elite level. There are several types of rugby boot, meant for players in different positions.