The word pyx (Lat., pyxis, which transliterates the Greek, pyxis, box-wood receptacle, from pyxos, box-tree) was formerly applied in a wide and general sense to all vessels used to contain the Blessed Eucharist. In particular it was perhaps the commonest term applied to the cup in which the Blessed Sacrament actually rested when in the Middle Ages it was suspended above the altar. Thus the Custumal of Cluny in the eleventh century speaks of the "deacon taking the golden pyx (auream pyxidem) out of the dove (columba) which hangs permanently above the altar". In later times however it has come about that the term pyx is limited in ordinary usage to that smaller vessel of gold, or silver-gilt, in which the Eucharist is commonly carried to the sick. Such vessels are sometimes made flat like a watch, sometimes mounted upon a little stand like a miniature ciborium. From the resemblance in size and shape the word pyx is also used to denote the small silver vessel or custode in which the Sacred Host is commonly kept in the Tabernacle, that it may be transferred thence to the monstrance when the Blessed Sacrament is exposed for the service of Benediction. In the Middle Ages pyxes for carrying the Eucharist to the sick were not unfrequently made of ivory. In spite of synodal decrees it is to be feared that there were many churches both in medieval and later times which preserved no proper pyx for taking Viaticum to the sick. In these cases the custom seems to have prevailed, even if it was not officially tolerated, of carrying the Host wrapped in a corporal in a burse which was suspended round the priest's neck or even of placing it between the leaves of a breviary.
The "pyx-cover", or "pyx-cloth", of which we sometimes read in medieval inventories, was a veil which hung over the pyx as it was suspended above the altar, and it was consequently a cloth of considerable size. At the present day the pyx when carried secretly to the sick, as is the case in most Protestant and many Catholic countries, is generally carried in a burse or pyx-bag, i.e. a silken bag suspended round the priest's neck within which the pyx is wrapped in a diminutive corporal used for that purpose.
- article from the Catholic Encyclopedia at www.newadvent.org