In the answers to this question about wing thickness, there were many references to wing thickness as a percentage (ranging from 6.5% to 13.5%).
What is that a percentage of?
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The wing thickness is basically the distance between the upper and lower surface of the wing (or airfoil).
The wing thickness is usually given as a percentage of the wing (or airfoil) chord(the distance between the leading and trailing edge). This is to make it a non-dimensional quantity for ease of comparison across different scales.
From the image in the answer you link to:
thickness in percent of chord
and the chord (length) is the distance between leading edge and trailing edge of the airfoil of the wing.