A <a href="https://en.wikipedia.org/wiki/shear_stress" class="extiw" title="w:shear stress">shear stress</a>, is applied to the top of the square while the bottom is held in place. This stress results in a strain, or deformation, changing the square into a parallelogram. The area involved would be the top of the parallelogram.
<a href="//commons.wikimedia.org/wiki/User:Krishnavedala" title="User:Krishnavedala">Krishnavedala</a>
2012-01-14
<span class="int-own-work" lang="en">Own work</span>