The "Golden Comma".
The comma was used as a "high power warning" - and persisted from the old days when it would be sent prior to throwing on the high QRO power so the near by operators wouldn't suffer hearing loss.
It was sent thus:
Daaaaaaaaaaaaaaah Daaaaaaaaaaaaaaah dit dit Daaaaaaaaaaaaaaah Daaaaaaaaaaaaaaaaaaaaaaaah
In later years - up until 1999 it was most often heard when someone was doing something stupid - like calling WCC - Chatham Massachusetts Radio - on 500 kHz during the daytime from the Mediterranean Sea, or calling ANYONE too damn much without letting up, or violating the SP - Silent Period.
Of course sending the comma was also a violation of the Silent Period - but this was generally overlooked - unless you heard a second comma which warned of the first comma being IN the silent period.