A series of tenterhooks

Tenter hooks were L-shaped staples, much like a bent nail, placed at regular intervals on a rectangular wooden tenter frame. When cloth emerged wet from the fulling process it was stretched out on these hooks, preventing it from shrinking as it dried - hence the phrase 'being on tenterhooks'.

The examples shown here are from Little Silver (left) and the quayside (right). They are shown below the famous illustration in Diderot's Encyclopedia of the mid 18th century showing clothworkers using tenter frames.