ISO metric screw thread

Main article: ISO metric screw thread
The basic principles of the ISO metric screw thread are defined in international standard ISO 68-1 and preferred combinations of diameter and pitch are listed in ISO 261. The smaller subset of diameter and pitch combinations commonly used in screws, nuts and bolts is given in ISO 262. The most commonly used pitch value for each diameter is known as the "coarse pitch". For some diameters, one or two additional "fine pitch" variants are also specified, for special applications such as threads in thin-walled pipes. ISO metric screw threads are designated by the letter M followed by the major diameter of the thread in millimeters, e.g. "M8". If the thread does not use the normal "coarse pitch" (e.g., 1.25 mm in the case of M8), then the pitch in millimeters is also appended with a multiplication sign, e.g. "M8×1" if the screw thread has an outer diameter of 8 mm and advances by 1 mm per 360° rotation.
The nominal diameter of a metric screw is the outer diameter of the thread. The tapped hole (or nut) into which the screw fits, has an internal diameter which is the size of the screw minus the pitch of the thread. Thus, an M6 screw, which has a pitch of 1 mm, is made by threading a 6 mm shank, and the nut or threaded hole is made by tapping threads in a 5 mm hole.
Metric hexagon bolts, screws and nuts are specified, for example, in British Standard BS 4190 (general purpose screws) and BS 3692 (precision screws). The following table lists the relationship given in these standards between the thread size and the maximal width across the hexagonal flats (wrench size):

ISO metric thread	M1.6	M2	M2.5	M3	M4	M5	M6	M8	M10	M12	M16	M20	M24	M30	M36	M42	M48	M56	M64
wrench size (mm)	3.2	4	5	5.5	7	8	10	13	17	19	24	30	36	46	55	65	75	85	95

In addition, the following non-preferred intermediate sizes are specified:

ISO metric thread	M7	M14	M18	M22	M27	M33	M39	M45	M52	M60	M68
wrench size (mm)	11	22	27	32	41	50	60	70	80	90	100

Whitworth
Main article: British Standard Whitworth
The first person to create a standard (in about 1841) was the English engineer Sir Joseph Whitworth. Whitworth screw sizes are still used, both for repairing old machinery and where a coarser thread than the metric fastener thread is required. Whitworth became British Standard Whitworth, abbreviated to BSW (BS 84:1956) and the British Standard Fine (BSF) thread was introduced in 1908 because the Whitworth thread was too coarse for some applications. The thread angle was 55° and a depth and pitch of thread that varied with the diameter of the thread (i.e., the bigger the bolt, the coarser the thread). The spanner size is determined by the size of the bolt, not the distance between the flats.
The most common use of a Whitworth pitch nowadays is in all UK scaffolding. Additionally, the standard photographic tripod thread, which for small cameras is 1/4" Whitworth (20 tpi) and for medium/large format cameras is 3/8" Whitworth (16 tpi). It is also used for microphone stands and their appropriate clips, again in both sizes, along with "thread adapters" to allow the smaller size to attach to items requiring the larger thread.
British Association screw thread
Main article: British Association screw threads
A later standard established in the United Kingdom was the British Association (BA) screw threads, named after the British Association for Advancement of Science. Screws were described as "2BA", "4BA" etc., the odd numbers being rarely used, except in equipment made prior to the 1970s for telephone exchanges in the UK. This equipment made extensive use of odd-numbered BA screws, in order—it may be suspected—to reduce theft.
While not related to ISO metric screws, the sizes were actually defined in metric terms, a 0BA thread having a 6 mm diameter and 1 mm pitch. Other threads in the BA series are related to 0BA in a geometric series with the common factor 0.9. For example, a 4BA thread has diameter (6.0 x 0.9^4) mm and pitch (1.0 x 0.9^4) mm. Although 0BA has the same diameter and pitch as ISO M6, the threads have different forms and are not compatible.
BA threads are still common in some niche applications. Certain types of fine machinery, such as moving-coil meters and clocks, tend to have BA threads wherever they are manufactured.
Unified Thread Standard
Main article: Unified Thread Standard
The Unified Thread Standard (UTS) is most commonly used in the United States of America, but is also extensively used in Canada and occasionally in other countries. The size of a UTS screw is described using the following format: X-Y, where X is the nominal size and Y is the threads per inch (TPI). For sizes 1⁄4 inch and larger the size is given as a fraction; for sizes less than this an integer is used, ranging from 0 to 16. For most size screws there are multiple TPI available, with the most common being designated a Unified Coarse Thread (UNC or UN) and Unified Fine Thread (UNF or UF).