The cross staff was first described in the 14th century and is thought to have originally been derived from a land surveying instrument.  It was taken up by navigators about a century later and is one of the earliest known Western navigational instruments.

By moving one of the sighting vanes along the staff, angular measurements between the horizon and a celestial body can be read off the scale on the staff.  Different lengths of vane are used according to the angle being observed.

This relatively simple tool was the principle method of celestial navigation until the back staff came into use in the 17th century.

Cross staff's are usually made of wood allowing them to be made relatively quickly and cheaply.