Business Source Economical No. 10 Window Envelope - Single Window - #10 - 9 1/2" Width x 4 1/8" Length - 24 lb - 500 / Box - White - No. 10 window envelopes feature an economical side seam and a window with film cover. Oyster-white, corrugated, expandable box provides easy access to envelopes. 24 lb.