Wingspan

The wingspan of an airplane or a bird is the distance from the tip of one wing to the tip of the other. The wingspan of an aircraft is always measured in a straight line, from wingtip to wingtip, independently of wing shape or sweep.
To measure the wingspan of a bird, a live or freshly dead specimen is placed flat on its back, the wings are grasped at the wrist joints and ankles, and then the distance is measured between the tips of the longest primary feathers on each wing.