The instrument is made up of several layers which are riveted and screwed together. The outer surfaces are gilt and worn areas show copper as the underlying metal.
The obverse has a scale in the form of a quadrant with a rotating index rule. In the spandrel of the quadrant and on the decorative border is a set of decorative reliefs showing European and Ottoman geometers measuring and surveying with various forms of square, quadrants, dividers and cross staff .
The quadrant scale has a descriptive caption 'IN HOC QVADRANTE SVMMA OMNIUM FRACTORVM NVMERORVM QVI IN MILLE PARTIBVS CONTINGERE POSSVNT SINE OMNI CALCVLATIONE INVENITVR'. Its upper edge has a double scale labelled 'PVNCTA DISTANT: LOCORVM' of a type which is repeated throughout the instrument. The scale is of equal parts 0 to 200, divided to 1 and numbered by 5. Using the same divisions is a transversal scale 0 to 1000, numbered by 5, which in effect subdivides each unit of the 200 scale into 5 dots arranged diagonally.
Vertical lines traverse the quadrant from this upper scale; every fifth line is more heavily engraved and is numbered, to 200. The arc of the quadrant carries the same numbering and provides transversal divisions to 1000. This arc is marked 'PVNCTA ALTITVDINIS STATIONIS'. At the 45? point there is an empty hinge; the corresponding part is lost.
The index rule is marked 'SCALA GEOMETRICI CVRSORIS' and carries the same double scale of equal parts as the upper edge of the quadrant. The rule was once broken and has been repaired with a piece of brass.
The main feature of the reverse is a table titled 'TABVLA SCALAE GEOMETRICAE MILLE PVNCTORVM VMBRAE VERSAE ET RECTAE'. This has 10 sets of three columns headed 'PVN:' (1 to 1000), 'ZAL', 'BRV:'. The data in the table can be represented by the simple formula (PVN ? ZAL) + BRV = 1000 (with the exception of the final entry which reads 1000 0 0 rather than 1000 1 0). This was to be used in conjunction with the transversally divided scales of equal parts.
On the frame to the left of the main table is a similar table titled 'TABVLA SCALAE GEOMETRICAE DVCENTORVM PVNCTORVM VTRIVSQVE VMBRAE' where the points run 1 to 200 and the equivalent relationship (and exception) holds. This was to be used with the scales of equal parts running to 200.
Along the upper edge is another double scale of equal parts to 200 and 1000; the same type of scale is also placed on the right edge and again runs to both 200 and 1000.
A longer version of the scale (to 285 and 1425) appears on the rotating index rule, which pivots in the top left corner. Note that both this scale and that on the upper edge of the frame begin not at the central pivot point but at the circumference of the circular pivot piece of the rule. The rule has two threaded holes which were presumably for sights; there are circular wear patterns around the holes. The reverse has the (seemingly later) inscription 'REGVLA PERSPICIENDI'.
On the right of the table is a slot with a sliding piece of uncertain function - perhaps to clamp the rule in a particular position. Alongside the slot is the inscription 'SCALA DEFERENTIS QVI REGVLAM HYPOTHENVSA ADDVCIT ATQ<VE> ABDVCIT'.
Signed on the lower part of the frame in a cartouche: 'CHRISTOPHORVS SCHISSLER GEOMETRICVS AC ASTRONOMICVS ARTIFEX AVGVSTAE VINDELICORVM FACIEBAR ANNO DOMINI .15.79.'. The signature is flanked by two engraved vignettes each apparently showing an instrument maker at work, surrounded by tools and products.
There were originally two hinged pieces on the lower part of the frame. Only one survives; its form resembles a belt buckle.
Associated with the square is a level which entered the Bodleian library with the square. The instrument can stand on its base, which has two decorated folding circular sights with central pinholes. The arc has a scale of equal divisions 90 to 0 to 90, divided to 5 and 1, and numbered by 5; the angular length of this scale is about 73?. There is a decorated support for suspending a plumb bob.
Provenance: presented to the Bodleian Library in 1601 by Sir Josias Bodley; lent to the museum in 1952 (Accession no. 52-83).