A tandem wing aircraft usually involves two full-sized wings, both of which are full airfoils. Sometimes an aircraft of this configuration can look like a variation on the biplane, but is in fact very different. The forward wing is often technically a canard, fitted with elevators, but both forward and aft wings provide lift. In the case of the QAC Quickie the aft wing serves as horizontal stabilizer, but pitch control comes from the forward wing.
In the case where the rearmost tandem wing is effectively an oversize tailplane it is referred to as a "Delanne wing" - from Maurice Delanne, a French designer of tandem wing aircraft.
The difference between a tandem wing and a biplane has to do with the horizontal proximity of one wing to the other. In a biplane, the wings are horizontally close, so that the lift vector on each wing is in close proximity to each other (measured longitudinally). Because of their proximity, there is little difference between a biplane and a monoplane in the relationship between the lift vector and the aircraft's center of gravity (CG). In a tandem wing design the lift vectors on the two wings are spread far apart longitudinally.
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