Albert Barnes Commentary Acts 16:24

"who, having received such a charge, cast them into the inner prison, and made their feet fast in the stocks." — Acts 16:24 (ASV)

Thrust them into the inner prison. Into the most secluded and secure part of the prison. The cells in the interior of the prison would be regarded as safer, being undoubtedly more protected, and the difficulty of escape would be greater.

And made their feet fast in the stocks. Greek: 'And made their feet secure to wood.' The word stocks, for us, refers to a machine made of two pieces of timber, between which the feet of criminals are placed, and in which they are in this way made secure. The account here does not necessarily imply that they were secured in precisely this way, but that they were fastened or secured by the feet, probably by cords, to a piece or beam of wood, so that they could not escape.

It is thought that the legs of the prisoners were bound to large pieces of wood, which not only hindered them but were often placed so as to extend their feet a considerable distance. In this condition, it might have been necessary for them to lie on their backs. If this, as is probable, was on the cold ground after their severe scourging, their sufferings must have been very great. Yet in the midst of this, they sang praises to God.