In aviation, a sesquiplane is a variation on the biplane concept, where one wing (usually the lower) is significantly smaller than the other either in span, chord or both. The name means "one-and-a-half wings." The arrangement reduces interference drag between the wings whilst retaining some of the biplane's structural advantage. The 1920s Pander E is an example of an aircraft with a lower wing of exactly half the span and nearly one quarter (23%) of the area of the upper one. Some designs keep the upper and lower spans nearly equal for structural optimization, whilst reducing the lower chord, allowing near vertical interplane struts; probably the best known examples are the Nieuport military aircraft of the Great War. The later Waco Custom Cabin series proved to be a popular example in general aviation. Although the sesquiplane was a common layout the 1920s and '30s, it was superseded along with all biplanes by improvements in monoplane structural design, driven by the recognition of the importance of lift-induced drag, though the Australian-built Transavia PL-12 Airtruk agricultural aircraft is a sesquiplane designed in the 1960s.