In Philippi, what did the jailer fasten Paul's and Silas's feet in?

Answer: The stocks

After they were beaten, the jailer put Paul and Silas in the inner cell and fastened their feet in the stocks.

“And after striking them with many blows, they threw them into prison and ordered the jailer to guard them securely. On receiving this order, he placed them in the inner cell and fastened their feet in the stocks.”— Acts 16:23-24 (BSB)