Dahl's sign (also Thinker's sign or Target sign) is a clinical sign in which areas of darkened (hyperpigmentation) and thickened (hyperkeratotic) skin are seen on the lower thighs and elbows. It occurs in patients with longstanding severe chronic obstructive pulmonary disease. The sign occurs because patients with COPD tend to sit forwards with their arms resting on their thighs, leading to chronic erythema of the skin at the points of contact. Over time, haemosiderin released from red blood cells trapped in the skin is released causing a brown discolouration of the skin.