No carpenter's toolbox is complete without a box-beam level, but choosing the right one can be a difficult task. In this article, senior editor Justin Fink looks at how box-beam levels are made. The level starts with an extruded aluminum frame that is straight, flat, and strong. Some levels are ridged for strength, and some have beveled or rounded edges, which can make it difficult to mark an accurate line. Most vials are glued to the frame, either at the top, bottom, or side, although one manufacturer (Sola) uses a glue-free attachment process. Modern levels use a single vial, either tubular or block, and can be read in both standard and inverted positions. Levels are calibrated to an accuracy standard of 0.029°. When choosing a level, ask other carpenters about the levels they use, and also consider how much you will need to use your level. If it will be used infrequently, a lower-priced model might suffice. If it will be used often, it might be a good investment to buy a more durable higher-priced model.