"This is the Irish Republican Army. A bomb is set to go off at the train station in 40 minutes' time. The code word is Shamrock."
Or maybe it's Ballymena. Or Easter Rising. Or Michael Collins.
For more than two decades, the IRA has used a movable feast of code words to authenticate its bomb warnings. The words change from time to time, but insiders say their meaning is always clear: "This is the real thing."
Telephoned threats, typically to radio stations or newspapers, hospitals or churches, invariably carry an authenticating code.
In the IRA's current campaign to disrupt the British election, the code has also been used to report bombs that never were: hoaxes.
Anti-terrorist specialists in Northern Ireland say the IRA began using the code words in the early 1970s, when bomb threats were so common that police had no way of knowing which were serious and which were pranks. The practice later spread to Protestant militants, who devised authenticating codes of their own.
The guerrillas settle on the word or phrase unilaterally, specialists say, often choosing words with historic or political significance to their cause.
Words come to be accepted as authentic codes if their use precedes the events that they predict. They are changed once they become too widely known.