An investigation into the composite or colinear form of the Müller-Lyer illusion, with one shaft adjustable by subjects, was carried out using black-on-white and CRT displays. Three fin-angle and two fin-length conditions were examined for both configurations of this figure, i.e., arrow-headed and feather-headed standards. Adaptations of the Müller-Lyer figure were also studied. Framing effects influenced the extent of this illusion. A highly consistent illusion error for balanced-field conditions, based on the formula k L cos A can be predicted which relates closely to previous experimenters' observed illusory errors and gives a remarkably simple explanation for this illusion.