A tool has a distal operative portion and a proximal shank with a mounting portion, for mounting the tool in a holder. A substantial portion of the mounting portion has a cross-sectional shape with at least one and fewer than six 60-degree segments thereof having segment ends joined by a flat surface. The holder has a distal axial aperture or mounting hole for receiving the tool, the aperture having a cross-sectional shape which has 60-degree segments thereof having segment ends joined by a flat surface, but no more such flat surface segments than those corresponding to flat surface segments of the tool. The holder further has a ball-detent mechanism or other locking mechanism for engaging the mounting portion of the tool.