There are a lot of Union Flags around at the moment, fluttering on this long grey Bank Holiday weekend to celebrate Her Majesty's Diamond Jubilee. What does not appear to be common knowledge is that there is a right and a wrong way to fly the flag; as I've seen a good few the wrong way up!
The correct flying of the flag comes down to the history of the Union Flag. The flag of the United Kingdom originates to 1606 and the unification of England and Scotland under King James the first- the Cross of St George - a red cross on a white background (England,which at the time of the design of the flag included Wales, so St David doesn't get a mention) was joined with the Cross of St Andrew (a white saltire cross on a blue background).
The current flag dates to the Act of Union in 1800 which added Ireland to the Union forming the United Kingdom. This meant that St Patrick's Cross (a red saltire cross on a white background was added onto the flag). In order to maintain the precedence of the nations on the flag (St Andrew's cross was already behind St George's) the cross of St Patrick was counterchanged on St Andrew's cross.
This means that the flag is not symmetrical, and there is therefore a right and a wrong way to fly the flag. The correct way is with the broad white stripe of St Andrew's cross uppermost against the flag pole/mast.
To fly the flag the wrong way up is considered lèse-majesté (literally injured majesty), an offence against a reigning sovereign or state. The internet is full of citations that this is 'theoretically still a crime' although I can find no evidence of this other than the translation of lèse-majesté as the crime of violating majesty. It is certainly not a listed offence under the current Treason Act!
The first time I was aware you could fly the flag the wrong way up was when I read Jeffrey Archer's 'A Matter of Honour' in my teens; in which a the hero realises he is in the wrong car as the flag on the front of the car is flying upside down!