In electronics, TO-18 is a designation for a style of transistor metal case. The case is made of metal and is therefore more expensive than the similarly sized plastic TO-92 package used for other transistors. The name is from JEDEC, signifying Transistor Outline Package, Case Style 18.
The tab is located 45° from pin 1. The lead diameter is nominally 0.45 mm, and the overall width (not including the tab) is 5.4 mm.
Different manufacturers have different tolerances, and the actual form factor may vary slightly, depending on function.
Generally, the 3-lead TO-18 is used for transistors (which usually have three leads).
There are variants for diodes, photodiodes and LEDs that have only two leads; some have parabolic reflectors in the top of the case rather than a sealed, flat top. Diode lasers such as those found in CD players also are often packaged in TO-18 cases, these have the lens also integrated into the case. The flange at the base is also sometimes thicker for the laser packages.
Other types of metal cases containing integrated circuits have as many as eight leads.