The x-ray of the Girl with a Pearl Earring shows us that whoever applied its preparatory layers worked in a similar way. During priming, the canvas was probably laced into a wooden frame (like the one in fig.8c). The undulations (cusping) that we see along the edges of the x-ray correspond to the points where the strings were attached to the canvas (dots on fig. 8d). When the witter tightened the strings to make the canvas as taut as possible, it pulled at these points and made the edges of the canvas curve. The x-ray of The Girl also shows curved strokes from where the ground was applied more thickly with the priming knife, especially in the lower left.
After the ground was applied, the primed canvas was taken off the laced frame, pulled taut, and attached to a strainer: a rectangle made of four wooden bars. We don’t know whether Vermeer attached the canvas to the strainer himself. Very few 17th-century canvases have survived the last 450 years attached their original strainer. One Vermeer painting – The Guitar Player – is an exception. The edges of the canvas (the tacking margins) were folded over the sides of the stretcher and ‘nailed’ in with wooden pegs.