I found this information on Dutch Chesstrees on agesofsail.net:
Chesstree: A timber fitted on the outside of the hull, just below the gunwale. It had one or more holes with
internal rollers or pulleys through which the main tack or sheets were hauled from within board.
Here's a drawing of a Dutch fashion chesstree ornamented with overlapping scales. Notice that the image appears as a hole through the chesstree but the reference says that there were internal rollers or pullieys inside, i.e. an early form of a sheave.
Here's the link to the website:
Ship & Shipbuilding Terminology - Advanced Search: chesstree
Bob