We investigate an extended game with observable delay under duopolistic competition in affine supply functions. Firms use the intercepts of supply functions as their strategic variables. Best replies are downward (upward) sloping if the common slope of supply functions is sufficiently low (high). Accordingly, simultaneous (sequential) play is selected at the subgame perfect equilibrium when best replies are negatively (positively) sloped. There exists a unique value of the slope at which best replies are orthogonal and the choice between simultaneous and sequential play is immaterial.