A: A corporal is a linen cloth placed atop the altar cloth but beneath the chalice and paten used at Mass. Traditionally, the fabric is linen, as was that which wrapped the body of the Lord in the tomb; hence the name, too: corporal, from the Latin word corpus, “body.” The corporal’s purpose is to collect any fragments from the Eucharistic body of Christ during the Mass, which are then enfolded within the corporal at the end of the Communion Rite.
While the size of corporals may vary, an average-sized corporal may be around 18 inches square. The starched linen is then folded into thirds, and then thirds again. In this way, a folded corporal (if 18 inches square when unfolded) is about six inches square; when unfolded, it has nine smaller square sections (three by three) formed by the indents of the folds.
At the start of Mass, the folded corporal is either held in a burse or placed directly upon the top of the chalice on the credence table. At the preparation of the altar at the beginning of the Liturgy of the Eucharist, an acolyte or deacon or the priest himself unfolds the corporal in the center of the altar. Placing the folded corporal about six inches from the near edge of the altar, the corporal is first opened to the left, as one would open the cover of a book, and then to the right; three of the corporal’s nine square sections are now visible. Next, the corporal is opened away from the one unfolding it so that six of the nine sections are now visible. Finally, the remaining section is opened down toward the edge of the altar and the one unfolding it; now all nine sections are visible. (Of course, opening the corporal according to this accepted pattern presumes that it was folded properly at the end of the last Mass.)
Many corporals will have a cross embroidered onto them, sometimes in the center, but usually upon the bottom, center square of the nine squares. If the corporal has not been folded correctly and the cross ends up on either of the sides or the top section, the corporal is either refolded and then unfolded again properly, or else the corporal is turned while maintaining its flat contact with the top of the altar. The corporal is never lifted by its corners and rearranged, for this defeats its entire purpose—to collect any fragments of the Eucharist.
Before a corporal is laundered, any visible Eucharistic particles are consumed. It is then rinsed with water which is poured into the sacrarium (a sink set aside for this specific purpose with a drain leading directly into the earth from the sacristy; see GIRM, 334). The corporal can then be cleaned, starched, folded, and ironed for future use at Mass.