CTCSS stands for Continuous Tone Coded Squelch System. ( LOCK & KEY SYSTEM)
Interference-Elimination Codes, sometimes refer to Tone Squelch, TalkGroup or Privacy Code.
It works like Key and Lock on Door. Walkie Talkie transmits sub-audible tone (Key) and opens the door (Lock) on Receiver. For example: Group A on Ch1 no CTCSS, Group B on Ch1 Code1, Group C on Ch1 Code38. Group B won't be bothered by Group A (no Keys) C (wrong Key). Group A can hear all groups because no Lock (incoming door is open).
Keep in mind that this feature doesn't actually protect your privacy. As you see Group A who turned off CTCSS can hear all in the same channel. In addition, anyone who has chosen the same channel and code as you can still transmit to your radio.
CTCSS 38ch Standard Frequency Table
| CTCSS ch | Hz | CTCSS ch | Hz |
| 1 | 67.0 | 20 | 131.8 |
| 2 | 71.9 | 21 | 136.5 |
| 3 | 74.4 | 22 | 141.3 |
| 4 | 77.0 | 23 | 146.2 |
| 5 | 79.7 | 24 | 151.4 |
| 6 | 82.5 | 25 | 156.7 |
| 7 | 85.4 | 26 | 162.2 |
| 8 | 88.5 | 27 | 167.9 |
| 9 | 91.5 | 28 | 173.8 |
| 10 | 94.8 | 29 | 179.9 |
| 11 | 97.4 | 30 | 186.2 |
| 12 | 100.0 | 31 | 192.8 |
| 13 | 103.5 | 32 | 203.5 |
| 14 | 107.2 | 33 | 210.7 |
| 15 | 110.9 | 34 | 218.1 |
| 16 | 114.8 | 35 | 225.7 |
| 17 | 118.8 | 36 | 233.6 |
| 18 | 123.0 | 37 | 241.8 |
| 19 | 127.3 | 38 | 250.3 |
