The limb of the plate is engraved with a degree scale, divided to 10 and to 5, subdivided to one degree, numbered from 10 to 360 with 0 at east. A later renumbering is engraved over the previous one and consists of groups of divisions 10 to 45, numbered from 10 to 40. Further to the centre the same scale is numbered in four quadrants with 90 at east, by ten with '5' at the intermediate points. The initials of the eight main winds are engraved clockwise from the east 'P', 'M', 'T', 'G', '[cross]', 'S', 'O', 'L'. Further to the centre are engraved the full names of the cardinal points 'LEVANTE', 'Scirocco', 'MEZZOGIORNO', 'Garbino', 'PONENTE', 'Maestro', 'TRAMONTANA', 'Greco'. On eight direction lines radiating from the centre are engraved the initials '[cross]' (for the east), 'S', 'O', 'L', 'P', 'M', 'L', 'G' in hand type script. On one side of the central part of the brass plate is the 'umbra recta' part of a double shadow square, inscribed 'Puncta recta', divided to ten and to five, subdivided to one, numbered 60 to zero to 60 by ten and 12 to zero to 12 by one. The scale is inscribed 'Quantitates Angulorum figurarum Regularum'. The other side of the plate has a double shadow square, inscribed 'Scala Altimetra' and 'Puncta versa' in correct and specular writing. The scale is divided to ten and to five, subdivided to one, and numbered 10 to 60 on the 'umbra versa' sides and 10 to 1 on all sides.

In the centre of the plate is a compass, surrounded by a scale of degrees, divided in quadrants to ten and to five, subdivided to one, numbered from ten to 90 by ten.

The sights, sliding on an alidade rotating jointly with the scale around the compass, are now missing.

The instrument is kept in a leather case, simply decorated with a gold line, with a hinged lid and soft internal pad.

The instrument was bequeathed by Croft Lyons and is described in F. A. B. Ward, A Catalogue of European Scientific Instruments in the Department of Medieval and Later Antiquities of the British Museum (London, 1981), p. 100, no. 291.