The most important properties of shims are their ability to separate and adjust the spacing between items in construction. The thickness of a shim is chosen according to the depth of the space it is intended to maintain. The footprint is chosen according to handling and load distribution needs. A permanent shim should be sized to fit entirely within the joint, behind the finish materials and without interfering with any sealant that can be installed in the joint. Typically withstanding ten tons of compressed force, lightweight "U" shaped Bridge Fasteners are designed to fit around bolts and anchors. They are often used for aligning windows and curtain wall, for example. Their rounded shape allows greater flexibility in aligning the shim with no protruding corners. They are offered in eight sizes and four color-coded thicknesses. Supplied bulk packed in boxes.