The open square has an inscribed quadrant and a diagonal bar incorporating a central compass ring arranged to graze the inside of the quadrant arc. A universal compass mount beneath the magnetic compass carries the instrument on a brass cylinder head now set on a wooden display stand. The pivot for an alidade is set at one corner, where the square is shaped to accommodate it. Two sides of the square have geometrical quadrant scales 0 to 60 [actually marked '90'] to 0, divided to 15, subdivided to 5, to 1 with alternate hatching, and to 1/2, numbered by 5, marked 'VMBRA VERSA' and 'VMBRA RECTA'. The inscribed quadrant has a degree scale 0 to 90, divided to 5, subdivided to 1 with alternate hatching. The compass mount is marked with 16 divisions and 32 subdivisions, the four cardinal points being marked with abbreviated names, 'SPT', 'OR', 'MI', 'OC'. Beneath a glazed cover is a magnetic needle over a blue and gold representation of the sky, sun and stars, and a target meridian line. The pivoted alidade has two sights, each being a rectangular plate with a circular hole containing an upright pierced by a pin-hole. The pinholes are aligned with the fiducial edge of the alidade. The reverse of the square has a pivoted mount for a plumb-line.
