UNLABELLED: Proper spindle assembly requires the Kinesin-14 family of motors to organize microtubules (MTs) into the bipolar spindle by cross-linking and sliding anti-parallel and parallel MTs through their motor and tail domains. How they mediate these different activities is unclear. We identified two MT binding domains (MBD1 and MBD2) within the SIGNIFICANCE STATEMENT: Spindle assembly and organization utilize molecular motors that cross-link and slide anti-parallel and parallel microtubules. How individual motors moderate both active sliding and static cross-linking is not understood.Using biochemical reconstitution, the authors determined that the Kinesin-14 tail contains two independent microtubule binding domains. MBD1 with weaker microtubule binding facilitates faster anti-parallel microtubule sliding, whereas the stronger MBD2 mediates tight parallel microtubule cross-linking, which was important for spindle assembly.These findings provide a mechanism for how Kinesin-14s differentially control microtubule sliding and cross-linking and provide insight into how molecular motors can mediate the dynamic organization of microtubules in the spindle.