Abstract: A coupler assembly is installed between a driving shaft and a driven shaft to transmit rotary power and motion at constant velocity at all times during their relative positioning varying from zero degree in line to ninety degree angularity as selected throughout a hemisphere of possible radial shaft directions. In a preferred embodiment four pairs of yokes or links are equally spaced both radially and circumferentially about the inline centerline of the driving and driven shafts. Four yokes or links are rotatably, slidably and/or reciprocatably contained, in respect to either the driving or driven shaft, by positioning their rod ends partially confined within individual cylinders formed in an overall cylindrical body; in turn secured to either the driving or driven shaft.