Thread pitch standards are standard values for pitches of different screw thread sizes. A screw thread's pitch is the distance from one of the thread's ridges to the next equal ridge. Thread pitch standards are distinct from pitch diameter standards, which measure the distance from the center of the screw to its pitch line. Thread pitch standards are measured in threads per inch, while pitch diameter standards use inches.
A screw's pitch is closely related to that screw's lead. Lead is the distance along a screw's rotational axis that the screw traverses during one full turn of its threads. For most screws, lead and pitch are equal, as most screws wrap only one ridge around the screw's shaft. Some screws, however, wrap multiple different ridges around the shaft, and for these screws lead and pitch have different values.
International standards organizations assign thread pitch standards to different screws based on nominal diameter size. Nominal size is the common size engineers and builders use when referring to different types of screws. Nominal size is only the common value by which people refer to screws, and this value often differs from the screw's actual size.
A single nominal screw size can have several different pitch standard values based on how many threads per inch each individual standard screw has. Likewise, screws with the same nominal size and pitch value can have different pitch diameter sizes based on how shallow or deep their threads are.