X = Cross-Veins [see the following Table]; one very significant X-vein termed X, 4 (or the Discal X) in the middle of the wing connects the longitudinal V, 4 to the V, 3, which are thus to be identified even in very abnormal patterns.
The cross-veins and cells are symbolised by having attached to the letters X and O respectively the indicative and qualifying figures and letters belonging to the V upon which they rest, that is to say, the V (or part of V) which is the lower boundary.
TABLE OF WING CELLS AND VEINS
O, 1.—Costal Cell (undivided), or Costal Areolet.
O, 1a1.—The Humeral Cell }
O, 1a2.—Second Costal Cell } Costal Cell, when divided.
O, 1b.—Subcostal Cell }
O, 2.—Marginal Cell (resting on V, 2).
O, 3.—Submarginal Cell (resting on V, 3).