A 7 inch (17.8 cm) square box drawn on the table in balkline billiards , from the termination of a balkline with the cushion , thus defining a restricted space in which only 3 points may be scored before one ball must be driven from the area. It developed to curtail the effectiveness of the chuck nurse , which in turn had been invented to thwart the effectiveness of Parker's box in stopping long, repetitive runs using the anchor nurse .