Skin friction is caused by viscous drag in the boundary layer around the object. The boundary layer at the front of the object is usually laminar and relatively thin, but becomes turbulent and thicker towards the rear. The position of the transition point from laminar to turbulent flow depends on the shape of the object. There are two ways to decrease friction drag: the first is to shape the moving body so that laminar flow is possible. The second method is to increase the length and decrease the cross-section of the moving object as much as practicable. To do so, a designer can consider the fineness ratio, which is the length of the aircraft divided by its diameter at the widest point (L/D). It is mostly kept 6:1 for subsonic flows. Increase in length increases Reynolds number. With Reynolds no. in the denominator for skin friction coefficient's relation, as its value is increased (in laminar range), total friction drag is reduced. While decrease in cross-sectional area decreases drag force on the body as the disturbance in air flow is less. For wings of an aircraft, a decrease in length (chord) of the wings will reduce "induced" drag though, if not the friction drag.