The website AltCodes.org provides a comprehensive list of special symbols that can be typed with the Alt key and number pad combinations on a keyboard. Most of the codes yield foreign-language variations of existing letters, and some of them are images or typographical symbols.
To use the Alt keyboard symbols, hold down the Alt key and type the appropriate number. This process has to be done with the number pad, so some laptop keyboards may not support these codes.
For example, Alt + 236 yields aleph-null, the infinity symbol. A separate group of four-digit codes yields foreign letters. Capital E with an umlaut is typed with the keyboard code Alt + 0203.