Hastings' Dictionary of the Bible (1898)
STOCKS, the name of a machine or instrument by which the feet of prisoners are secured. Job 13:27; Ex 33:11. It is said that the jailer at Philippi, to whose custody Paul and Silas were: committed with a strict charge to keep them safely, not only put them in an inner prison or dungeon, but made their feet fast in the stocks. Acts 16:24. The upper half being removed, each leg is placed, just above the ankle, in the groove of the lower half, and then the upper part is so fastened down as to confine them inextricably. Ancient Stocks.
The "stocks" used on Paul and Silas could be turned into an instrument of torture by widely separating the legs. The "stocks" used on Jeremiah, Jer 20:2, were, properly speaking, the pillory, because the neck and arms as well as the legs were confined, and so the body was bent.