Limb dominance is a human behavioral characteristic with many cultural, practical, scientific, and clinical implications. Yet why the dominant limb performs better across a range of motor skill-requiring tasks remains unanswered. Is it because of an intrinsic hemispheric advantage or instead is it the result of life-long practice with the dominant side? We tested these alternatives using two tasks either cross sectionally or after training. The first was 3D reaching with either an inertial challenge or the need to use a stick-like tool. The second required participants to write with their dominant and nondominant elbows. We applied a geometric analysis to quantify movement-trajectory shape.