Recently it has become commonplace for the flag above Parliament to be flown at half mast. Some of Parliament's followers on Twitter and Facebook have asked why this is done. So, what does flying a flag at half mast represent and why is it such a potent symbol? A country in mourningWhen a national flag is flown at half mast, it shows that the country is in mourning. It can be for an important individual, such as a national leader. It can also be for many people, for example those killed in the recent terrorist attacks. The flag symbolises the country and flying it at half mast shows the whole country is grieving. Lowering the flag on top of Parliament shows that Parliament itself is in mourning. The recent terrorist attacks have been attacks on democracy itself. So it was appropriate that the flag above Parliament was lowered afterwards as a mark of respect. Lowering the ship's flagThe practice of lowering a flag to represent mourning or distress seems to have begun at sea. When the captain or a senior officer on a ship died, the rest of the crew would mark the loss by lowering the ship’s flag. It was lowered to half the height of the ship’s main mast and remained in that position until the ship returned to port. The Flag Institute promotes the use of flags and conducts research into their history. It suggests that lowering the ship's flag made room for an “invisible flag of death” above it. Who decides when the flag above Parliament should be lowered? The Department for Digital, Culture, Media & Sport decides whether or not to lower the flag on top of Parliament. It acts on the instruction or order of the Queen or other ruling monarch. Ironically, the monarch’s own flag, the Royal Standard, is never at half mast. That's because there's always a living monarch as the throne immediately passes to a successor when the monarch dies. An exception was made in 1997 for the funeral of Princess Diana. The Union flag replaced the Royal Standard above Buckingham Palace. As it was a national flag it could be lowered, to reflect the prevailing mood of the country. A nation’s flag is its most powerful and enduring symbol, and lowering it to half mast is the most striking use of it. It's tragic that flags have been at half mast so often recently. We can only hope that in the future they will be lowered far less often.