In which situation demand curve will be a rectangular hyperbola?

when elasticity of demand happens to be equal to unity

When elasticity of demand is 1 i.e. it is a unitary elastic demand curve, then it will form the shape of a rectangular hyperbola. Rectangular hyperbola is a curve under which all rectangular areas are equal. When the elasticity of demand is equal to unity (e_d = 1) at all points of demand curve, then the demand curve is rectangular hyperbola.