A tap and die drill chart specifies the appropriate size of drill bit to use to create an optimally sized opening for use with a particular tap, and the correct size of die to complement the tap dimensions. These charts are important to create accurate, matching pairs of male and female threads in order to tightly secure them together. Often, corresponding taps and dies are sold together, so the chart is only necessary for the drill size.
A tap is a tool used to create a female screw thread, such as the threading on the inside of a nut. There are three common types of tap: bottoming, plug and taper. A bottoming tap has almost no taper on the end, making it useful for tapping close to the bottom of a hole. Plug taps and taper taps both have tapers on the end, making them suitable for starting a tap. A taper tap is smaller than a plug tap, with a more pronounced taper, giving it an advantage in tapping difficult materials.
A die is a tool used to create a male screw thread, such as the threading on the outside of a bolt. It generally has a small split in the side and an adjusting screw, which allows it to be slightly altered in size for different materials or degrees of sharpness.