Harry Thurston Peck, Harpers Dictionary of Classical Antiquities (1898)

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Plethrum

（πλέθρον).

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1.

A measure of length among the Greeks=1/6 of a stadium=100 Greek feet=little more than 101 English feet, or 33 yards 2 feet.

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2.

A unit of square measure, the square of 100 Greek feet, or 10,000 Greek square feet—i. e. an area of the extent of 10,226.2656 square feet, or about 1136.24 square yards—i. e. about two perches less than a rood, or quarter of an acre. See Mensura.