Code names and acronyms were vital to help maintain the blanket of secrecy in the planning and build-up to the Allied invasion of Europe in 1944.

Here are a few:

:: D-Day: The date of the operation.

:: H-Hour: The hour of the invasion :: Bolero: The build-up to D-Day in Britain.

:: Operation Overlord: The overall invasion plan.

:: Operation Neptune: The seaborne invasion.

:: Mulberry: Artificial harbours towed across the Channel

:: Ham and Jam: The signal indicating the bridges at Benouville (Pegasus Bridge) and Ranville were secured by Allied forces.

:: Utah, Omaha, Gold, Juno and Sword: Code-names for the five landing beaches in Normandy on D-Day.

The words ''doom'', ''debarkation'' and ''deliverance'' have all been suggested as meanings for the ''D'' in D-Day.

But the letter is derived from the word ''Day'' and means the day on which a military operation begins.

D-Day has been used for many different operations but is most closely associated with the Allied landings on Normandy's beaches on June 6 1944.

The day before D-Day was D-1 and the day after was D+1.

It meant that if the date for an operation changed, military planners would not have to change all the dates in their plan.

Such a thing happened for the Normandy landings D-Day, which was originally planned for June 5 1944 - but bad weather delayed it by a day.