The instrument consists of a brass plate with a hole in the centre carrying a sliding disc. On one side of the square plate is a partly deleted inscription (probably modern), reading 'MARTIN FREY'. A museum accession card reports that the inscription continues 'REGENSPURE 1590' but this is no longer legible.
Around the sliding disc is an equal scale of latitudes from [0?] to 90? by 2?, numbered by 10?. A ruler marked with an unequal scale from 1 to 12 by 1 is retained by means of two slots. On the other side of the square plate is a calendar scale with months and declination of the sun indicated by zodiac signs.
One side of the round plate is engraved with latitudes from [0?] to 90? by 2?, numbered by 10?, a shadow square, the declination of the sun indicated by the symbols of the zodiacal signs, and has a counterchanged alidade. The other side of the round plate has a lunar volvelle with an hour scale marked 1 to 12 twice and 1 to 24.