Morse code (commonly referred to as "CW" or continuous wave) is the oldest mode used on the air^{[citation needed]}and exists as a digital encoding of the Latin alphabet. It first came into prominence in amateur radio through the use of Spark gap transmitters, later being transmitted via continuous sine waves produced by electronic oscillators being keyed on and off. This continuous sine wave signal is the namesake of the mode's abbreviation of CW.

The amateur radio bands are the last places on the radio spectrum where Morse Code and CW are still used.^{[citation needed]} It has several advantages, even against modern digital modes. CW has an extremely low bandwidth, on the order of tens to hundreds of Hertz, roughly on par with ultra-low-bandwidth digital modes. It is only in the past decade that weak-signal modes have had weak-signal performance comparable to CW, and even then the ability of a skilled human operator to use context and human pattern-recognition ability to decode CW cannot be utilized with other modes. CW can also be sent and received using extremely simple equipment, making it popular with Homebrewers and SOTA participants.

Fundamentals

A Morse code sequence is composed of a short signal (usually · or "dit") lasting one time unit, and a long signal (usually - or "dah") lasting three time units or three dits. There is a one-unit pause between dits and dahs, a three-unit pause between characters, and a seven-unit pause between words. Morse code uses variable-length encoding such that more common characters have shorter encodings than less-common characters. There are no separate sets of upper- and lowercase letters.

Prosigns (an abbreviation of "professional signs") are used on CW for message encoding and flow-control purposes. They are composed of long strings of dits and dahs, and written as letter sequences that match their encoding with an overline above them. For example, the prosign K—"over to you"—is sent as -·-, and the prosign KN—"over to you only"—is sent as -·--· without any inter-letter spaces. Other common prosigns include AR, ·-·-·, used at the end of calls and responses to calls; SK, ···-·-, at the end of a contact; CL, -·-··-··, used after SK to indicate that the operator is closing down their station and will not respond to further calls; and HH, ········, used to indicate that an error was made. DE, used before the transmitting station's callsign, looks like a prosign but is sent with normal spacing. At the end of a contact, E E is often sent with drawn-out letter spaces. Q signals (QSO, QRZ, etc.) perform a function similar to prosigns but are sent normally, with inter-character spaces. When used to ask a question (e.g. QRL?—"Is this frequency in use?"), a question mark (··--··) is appended to the Q signal.

Tables

These tables show the internationally-standard variant of Morse code as used on the radio. Other variants of Morse were developed in the past, but none are used today.

**Letters**
Symbol	Morse Code
`A`	`· -`
`B`	`- · · ·`
`C`	`- · - ·`
`D`	`- · ·`
`E`	`·`
`F`	`· · - ·`
`G`	`- - ·`
`H`	`· · · ·`
`I`	`· ·`
`J`	`· - - -`
`K`	`- · -`
`L`	`· - · ·`
`M`	`- -`
`N`	`- ·`
`O`	`- - -`
`P`	`· - · ·`
`Q`	`- - · -`
`R`	`· - ·`
`S`	`· · ·`
`T`	`-`
`U`	`· · -`
`V`	`· · · -`
`W`	`· - -`
`X`	`- · · -`
`Y`	`- · - -`
`Z`	`- - · ·`

**Digits**
Symbol	Morse Code
`1`	`· - - - -`
`2`	`· · - - -`
`3`	`· · · - -`
`4`	`· · · · -`
`5`	`· · · · ·`
`6`	`- · · · ·`
`7`	`- - · · ·`
`8`	`- - - · ·`
`9`	`- - - - ·`
`0`	`- - - - -`

**Special Characters**
Symbol	Morse Code
`.`	`· - · - · -`
`,`	`- - · · - -`
`:`	`- - - · · ·`
`;`	`- · - · - ·`
`?`	`· · - - · ·`
`!`	`- · - · - -`
`-`	`- · · · · -`
`_`	`· · - - · -`
`(`	`- · - - ·`
`)`	`- · - - · -`
`'`	`· - - - - ·`
`=`	`- · · · -`
`+`	`· - · - ·`
`/`	`- · · - ·`
`@`	`· - - · - ·`