ROUND FILTER, SQUARE HOLDER
Polarisers are circular by their nature, as they need to have a rotating front element in order to let you control the strength of the polarising effect. But as well as coming as the circular screw-in variety, they’re also widely available for square filter systems, and are usually fitted via an adaptor that’s attached to the front of the filter holder, or via a caddy that slots into the holder and allows the filter itself to be rotated.