Textron Fastening Systems developed the Torx bits and drivers as a way to reduce cam out, where the driver slips out and potentially damages the bit. The Torx system was also originally used for security purposes since Torx screws couldn't be removed with normal tools and few people had access to Torx drivers.
Torx Socket Screws
Torx socket screws have a six-pointed star indentation that the Torx driver fits into. The smallest Torx socket, measured across the star from point to point, is called the T1. The T1 measures .031 inches across and has a maximum allowable torque range of .02 to .03 Newton-meters, or N m. The Torx sizes increase by adding one to the T number, like T2, T3 and T4, up to T10 which measures .107 inches and has a maximum torque range between 3.7 and 4.5 N m. The Torx numbers then increase by 5, like T15, T20 and T25, but excluding T35 and adding T27, up to T60 which measures .519 inches and has a torque range between 379 and 445 N m. The series then increases by 10, up to the largest Torx socket, T100 which measures .871 inches and has a torque range of 1843 to 2048 N m.
Torx Head Bolts
Torx bolts are the opposite of the socket screws as these have a star-shaped protrusion that a driver fits around. These bolts are named with an E, followed by a number. The smallest bolt is E-4, which measures .15 inches from point to point. The E numbers increase by one up to E-8, which measures .29 inches across. The numbers then increase by 2 up to E-20, which measures .72 inches across. The largest bolt is E-24, which measures .87 inches across.
Torx Screw Sizes
The diameter of the screw head that a Torx indentation can be created on will depend on the type of screw. A #2 screw, which measures .0625 inches across, will fit a T3 Torx socket if it is a socket set screw, but if it is a flathead screw, it will take a T6 socket. Each type of screw will take a slightly different socket size (see Resources).
Textron created the Torx Plus set of drivers and sockets to increase the amount of maximum torque and reduce the occurrence of cam out. The Torx plus have the same measurements as the normal Torx sockets do, but whereas a T4 has a torque range of .22 to .28 N m, the Torx Plus version of that socket, called 4IP has a torque range around .44 N m. This increase of torque range happens across the board for every Torx socket size. The Torx plus sockets also have slightly different screw sizes (see Resources).