LH

LH stands for left hand and denotes a component, tool, or assembly configured or oriented for operation or installation from the left side. In manufacturing and mechanical trades, this marking distinguishes mirror-image pairs of identical-looking parts that are not interchangeable. A left-hand door hinge, for example, swings differently than its right-hand counterpart, even though both perform the same function.

The term appears most often in aerospace, automotive, and machinery contexts where symmetrical assemblies require left and right versions. A turbine blade marked LH mounts on the left side of a rotor and may have a different pitch or curvature than the corresponding RH blade. Similarly, directional equipment such as screw conveyors, spiral drill bits, and helical gears each come in left-hand and right-hand rotational variants. Without clear labeling, installing the wrong version causes assembly failure or unsafe operation.

The confusion risk is real. Parts that look nearly identical during manufacture and storage can end up on the wrong side of an assembly, particularly when workers are under time pressure or handling thousands of similar components. Some shops stamp or paint LH and RH directly onto parts; others track the distinction only in bills of material and work instructions. High-value or safety-critical assemblies warrant separate storage and fixture design to prevent mix-ups.

The convention varies slightly by industry and geography. In some European trades, the notation may appear as G (German links) or D for right. In plumbing and HVAC, the terms left-hand and right-hand typically describe the approach to a valve or fitting when facing it from a standard orientation. Always verify the specific convention in your assembly documentation, as assuming the standard left-right definition has led to costly rework.