A cross-staff is a surveyor’s instrument used to determine right and other angles, available with a open, cylindric, octagonal, hexagonal or spherical body, sometimes topped by a compass which would be used to take the bearing of the line.

The octagonal form is known as a French cross-staff. It consists of an octagonal brass tube with slits on all eight sides. It has an alternate vertical slit and an opposite vertical window with a vertical horse hair, affine wire or sight cord on each of the four sides. These are used for setting out right angles. On the other side are vertical slits for setting out angles of 45 degrees. The base carries a socket so that it may be fitted on the pointed staff when the instrument is to be used.

Research would suggest that these octagonal variations of the cross-staff were manufactured in France by companies such as Lerebours et Secrétan, and offered in the UK by retailers including Casella and Stanley.