Stress that acts ALONG a member is normal stress. Stress that tries to slide one face past another is shear: $\tau = \dfrac{V}{A}$ — tau equals V over A. $\tau$ (tau) is the average shear stress; $V$ is the transverse force trying to slice the part; $A$ is the area in shear, the surface that would actually have to fail.

Which makes counting planes the real skill. A pin in single shear has one section to fail on. A pin in double shear — held in a clevis, loaded in the middle — has two, so they share the load and the stress halves. Counting one plane where there are two overstates the joint; counting two where there is one is the dangerous direction, and it is the reason this lesson makes you say which it is before you divide.

Two companions travel with it. $G = \dfrac{\tau}{\gamma}$ — G equals tau over gamma — is the shear modulus, where $\gamma$ (gamma) is the shear strain: the angle, in radians, that a right angle gets pushed out of square. And $\nu = \dfrac{\varepsilon_{lat}}{\varepsilon_{ax}}$ — nu equals epsilon-lat over epsilon-ax — is Poisson's ratio: pull a bar and it gets thinner, and $\nu$ is how much thinner per unit of stretch. The subscripts say which strain is which — lat is the sideways one, ax the lengthwise one you pulled. For metals $\nu$ sits near 0.3, and it can never pass ½: a material that gave back more sideways than it took lengthwise would gain volume for nothing.