important, or common.
* Two-letter signals for other messages, sometimes followed with a numerical "complement" which supplements or modifies the message.
* Three-letter signals beginning with "M"; these are the Medical Signal Codes.

In some cases, additional characters are added to indicate quantities, bearing, course, distance, date, time, latitude, or longitude. There is also provision for spelling words and for indicating use of other codes. Several of the most common single-letter signals are shown at the right. Two-letter