Three principal rings, the outermost, meridian ring having an adjustable connection to a suspension ring, arranged to fit into a set of screw holes and to be clamped by a screw at different points within part of a latitude scale on the meridian ring, 0 to 90 to 0, divided to 10, subdivided to 5 and to 1, numbered by 10.
The equatorial ring is divided into hours 0 to 12 twice (half on each side of the ring), divided to 1 hour, subdivided to 20 minutes and to 4 minutes. The equatorial ring is engraved: 'ALBERTO . IMPERATORIS . FILIO . IMP : FRATRI [star] ISABELL? . PHILIPPI . REGIS. FILI? . PHIL : REGIS . SORORI . ?TERNVM . REGERE . IA . D . SVCCA'
The declination ring has an adjustable pinhole moving over a scale of months. Month names are marked in Latin (sometimes abbreviated) and divided to 5 and to 1 day, not numbered. This scale is distributed on either side of the equatorial ring according to the solar declination, and when the rings are folded, the meridian and equatorial rings each carry half of an equivalent scale marked with the signs of the zodiac with each sign divided into 6 (i.e. to 5 degrees). The declination angles for these scales are subtended at a point on the opposite side of the declination ring (close to its intersection with the equatorial ring), i.e. not alt the centre of the rings. At this point there is a pinhole that acts as a target for the light passing through the adjustable pinhole. The plate for the adjustable pinhole is shaped, decorated with sun, moon and stars, and bears the motto 'Simul et Semper' and a monogram of the letters 'A', 'B', 'I' (?), 'C' and 'E', surmounted by a coronet. The plate is carried by a mount moving in a slot in the declination ring and is adjusted against the declination scale.
At two points the rim of the equatorial ring is marked 'Ante Meridiem'. The meridian ring is marked twice with the dates of the equinoxes. The declination ring is engraved with various devices and decorations, including an eagle, the initial A, the monogram and two burning hearts. On the outside of the declination ring is an index marked 'LINEA HORARIA', in the form of an engraved snake and arrow, where the time is noted on the equatorial ring.
See: J. Bennett, The Measurers: a Flemish Image of Mathematics in the 16th Century (Oxford, 1995), pp. 52-3 and K. van Cleempoel, et al, Instrumentos Cientificos del Siglo XVI: la Corte Espa?ola y la Escuela de Lovaina (Madrid, 1997), p. 215.