A tool (1) has a distal operative portion and a proximal shank (2) with a mounting portion (3), for mounting the tool in a holder (10). 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 (5). The holder has a distal axial aperture or mounting hole (12) for receiving the tool (1), the aperture having a cross-sectional shape which has 60-degree segments thereof having segment ends joined by a flat surface (13), but no more such flat surface segments than those corresponding to flat surface segments of the tool. The holder further has a ball-detent (21) mechanism or other locking mechanism for engaging the mounting portion (3) of the tool (1).