Shear strength is the amount of force that a fastener can withstand when applied from the sides. A nail is more pliable than a screw. This implies that when forces are applied to the sides of a nail, the nail can somewhat flex to meet these pressures. A screw, on the other hand, has relatively low shear strength. If too much force is applied to one side of a screw, it will break. The shear strength of a screw is determined by how thick it is and what type of material it is made of. If two screws of equal length are compared, the one with greater cross-sectional area will have greater shear strength. For example, if four 1/4 inch screws are used to attach two pieces of wood together, then the screw with the greatest cross-sectional area (the #1 screw) will be the last one attached and likely to fail first. The reason for this is that as you move down the screw list (#4 screw first, then #3, etc.), each screw has the same length so their relative thicknesses are the same. However, the cross-sectional area of a screw increases as its diameter decreases. So, the #1 screw has the greatest cross-sectional area and will likely fail first if too much stress is applied to it. Screws are designed to resist pull out rather than push in. Therefore, they should not be used as anchors for hanging things from your wall or as stand-alone supports.