About Stringers

In board design, the “stringer” is a board’s central plane of reflection, down the middle of its deck and its keel. In construction, the stringer can have no special parts, or can embed a stiff, thin, vertical slat, usually of wood but sometimes of carbon fiber, running from nose to tail. The stringer serves to increase the board’s overall strength and reduce its flexibility. Some boards have multiple stringers.”

Wikipedia, https://en.wikipedia.org/wiki/Surfboard#Stringer