Adam Clarke on Acts 16:24

The inner prison - Probably what we would call the dungeon; the darkest and most secure cell.

Made their feet fast in the stocks - The το ξυλον, which we here translate stocks, is supposed to mean two large pieces of wood, pierced with holes like our stocks, and fitted to each other, that, when the legs were in, they could not be drawn out. The holes being pierced at different distances, the legs might be separated or divaricated to a great extent, which must produce extreme pain. It is this circumstance to which it is supposed Prudentius refers, in speaking of the torments of St. Vincent: -

Lignoque plantas inserit,

Divaricatis cruribus. "They placed his feet in the stocks, his legs greatly distended!"

If the apostles were treated in this way, lying on the bare ground with their flayed backs, what agony must they have suffered! However, they could sing praises notwithstanding.