Torx head sizes are described using the capital letter "T" followed by a number ranging from T1 to T100.  A smaller number corresponds to a smaller point-to-point dimension of the screw head (diameter of circle circumscribed on the cross-section of the tip of the screw driver). Common sizes include T10, T15, and T25, while , T35, and T47 tend to see specialized use. Only the proper driver can drive a specific head size without risk of damaging the driver or screw. The same series of Torx drivers is used to drive SAE , metric and other thread system fasteners, reducing the number of bit sizes required.