Proper allowance shall be made for _____, contraction, jarring and vibration.

The correct answer is linked to the physical behavior of materials under varying conditions. In mechanical systems, particularly those involving ducts, piping, or any form of structural elements, materials tend to react to temperature changes, which causes them to expand or contract.

Allowing for expansion is critical to avoid damage, leaks, or failures that could occur if these natural movements are not accounted for in the design and installation of mechanical systems. For example, if a pipe heats up due to the fluid inside reaching a high temperature, it expands. If there is no allowance for this expansion, it could lead to stress on the joints or even cause the pipe to burst.

While movement, displacement, and pressure are relevant to mechanical systems, they do not specifically capture the need for accounting for changes in size or volume due to temperature variations. Therefore, the focus on expansion highlights a fundamental principle in mechanical design that enhances the reliability and safety of the system.