Dr Ian Plummer

http://www.oxfordcroquet.com/laws/6th/10.asp
Oxford Croquet

10. BALL OFF THE COURT

A ball leaves the court as soon as any part of it would touch a straight edge raised vertically from the boundary. It then becomes a ball in hand and an outside agency. The striker must consult the adversary before testing whether or not a ball is off the court if the position is critical.