ASCII Encryption

The ASCII option used regular alphanumeric characters.
If using the Hex format however, it was either selected with a button in the configuration or the user had to start the key with "0x" (that's the number zero and a lower-case "x").

This method generates a Hexadecimal key from the ASCII string that was entered.
Note that this is not the same as directly entering an ASCII key. 

Below is a list of the different number of characters you need when using either a HEX or ASCII WEP key:

One ASCII Character is 8 bits
One HEX Character is 4 bits
40-or 64-bit ASCII WEP code has 5 characters
40- or 64-bit HEX WEP code has 10 characters
128-bit ASCII WEP code has 13 characters
128-bit HEX WEP code has 26 characters

Generally, 802.11b supports 64-and 128-bit encryption; 802.11b+ (enhanced) supports 64-, 128- and 256-bit encryption;
802.11g supports 64- and 128-bit, and 802.11A supports 64-, 128- and 152-bit encryption.

Some vendors will also offer stronger encryption levels. D-Link for example, allows the option of using 256-bit encryption (58 characters), providing that only a D-Link wireless device is being used.