From
Flag FAQ
Why is the red cross inside the white one on the Union Jack off-centered? Is it possible to fly a Union Jack upside-down?
A brief explanation: the "X" portion of the Union Jack is a combination of the Cross of St. Patrick (the red one) and the Cross of St. Andrew (the white one), plus a white fimbriation added on either side. St. Andrew's is on the right in every quarter (as you turn the flag clockwise) due to a rule of heraldry giving Scotland a position of precedence (on the right) over Northern Ireland as the more senior member of the United Kingdom. Thus, the flag is not symmetrical, and it is possible to hoist upside-down. British Scout troops are taught to look at the first quarter of the flag (in the upper hoist) and think "White on right!" in order to make sure it's raised correctly.