Manufacturers and retailers use a number of different systems to measure the size of cotton. One international standard of thread measurement is the Tex system. The finer the thread, the smaller the size number and the coarser the thread, the larger the size number. A thread sold as T40 weight is a medium- to heavyweight thread that is often used for quilting.
Originally, cotton had no fixed sizing system and buyers simply chose the thread they preferred by look and feel. Over the course of the 19th century, people developed several sizing systems. For instance, in 1912, cottons were sized by number with higher numbers indicating a finer thread. Silk threads were sized by letter. Threads could also be listed using special terms that referred to the number of strands and type of twist.
Several thread sizing systems are currently in use. The international standard used by most industrial manufacturers is the Tex system. Tex thread sizes are given as single numbers or as numbers preceded by the letter T, such as T40. This system may not be in use in fabric stores and hobbyist shops, however. Many of them still use systems such as the equivalent size method or the denier system. Crafters should take care to determine which system is in use before choosing a thread.
In the Tex system, thread numbers are based on the weight in grams of a piece of thread a kilometre long. Higher numbers indicate a heavier thread. Therefore, a kilometre of T10 thread would weigh ten grams while a kilometre of T100 thread would weigh 100 grams. One kilometre of T40 thread weighs 40 grams, which is roughly equivalent to a thread rated 400 in the denier system, 22 in the metric system, or 50/3 according to the cotton count system.
Differences in the manufacturing process can introduce small irregularities in the thread produced. Listing all thread by its exact size and weight would be time- consuming and inconvenient. In the Tex system, thread is usually rounded off to the closest relative size, so T40 thread could actually be anywhere from Tex size 40 to Tex size 49.9. This difference is minimal and will not affect most sewing.
T40 thread is a medium to heavy weight and suitable for use on fabric between six and eight ounces per yard in weight. It works best with a needle size between 90 and 100 and is appropriate for general utility sewing. T40 thread is a poor choice for light, airy fabrics such as gauze, as it may weigh them down or cause them to pucker.