A trestle comprises two pairs of legs (12a,12b,13a,13b) which can be opened and closed relative to one another. A beam (10) has two depending poles (11) and each pole is clamped by a clamping mechanism (14a,14b) carried by an associated pair of legs. When the legs (12a,12b,13a,13b) are open, the clamping mechanism (14a) is operative to hold the associated pole (11) against movement relative to the associated legs (12a,12b) whereas when the legs are moved together, this releases the clamping mechanism and allows the position of the pole to be adjusted.