|At the most general level, a compendium is simply an instrument made up of several disparate elements. An astronomical compendium is a mathematical instrument which has a range of astronomical functions incorporated into a single small device.|
As a catch-all term there is no standard list of functional elements, but sundials, compasses, calendars, maps, astrolabes, quadrants and various tables are often to be found. Neither is there a standard form for the resulting instrument; astronomical compendia are found in many shapes and formats, whether rectangular, circular, oval or octagonal, and are sometimes constructed to mimic other artefacts, such as books, with covers and leaves.
Astronomical compendia seem to have been particularly favoured in the 16th century, and many of the surviving examples are finely gilt and engraved, indicating that they were luxury items of ornament and display for a relatively wealthy clientele. Certainly, the component astronomical elements were too small for precise observational use. Moreover, the presence of suspension rings on several examples suggests that they might have been worn either on a chain around the neck or hanging at the waist from a belt.
The simpler compendia have just two folding leaves and the boundary between compendia and diptych dials can be a hazy one, sometimes seemingly dictated more by the material in which the instrument is made (brass for compendia and ivory for diptych dials) than the instrumental features which the instrument incorporates.