This tool holder is designed to fit over the mounting post of an Olympus SZ51 or SZ61 stereomicroscope, although it should fit on any microscope that uses a 1.5 in post. Quick testing shows that it also fit (although not as snugly) onto older post mounted Zeiss stemi stereomicroscopes. The holders are sized to fit the handle of an xacto knife or pen, but not quite big enough to fit a sharpie or marking pen.