What is the correct way to refer to a lag, screw or bolt?
Although these terms are used interchangeably, lags should technically be referred to as a screw and not as a bolt.
A bolt is a fastener with machine thread that can accept a nut. A bolt is properly assembled and tightened by rotating the nut. A screw, on the other hand, is a fastener that is properly installed by rotating the head of the fastener. Since lags are not used with a nut and installed by turning the head of the fastener, the proper technical term would be “lag screw”. Lag screws can be provided with either a hex or square head.
A bolt is available with many different thread types including Unified National Coarse, Unified National Fine, 8 Unified National, Acme, metric, fetter, and lag. Both machine bolts and lag screws are made to the requirements of ASME B18.2.1. ASME, which is the standards organization specifying dimensions for construction fasteners, also refers to lags as screws rather than bolts. Lag screws are almost always used in wood applications.