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A small plate that is use to support cups, more specifically teacups.
The saucer is useful to protect surfaces from possible damage due to the heat of a cup, and to catch overflow, splashes, and drips from the cup, so protecting both table linen and the user sitting in a free standing chair who holds both cup and saucer. It can also be an area to place your damp spoons.
Often comes along with a tea set especially when being served to others.
See also
The following tags implicate this tag: holding_saucer (learn more).