The flag of the United Kingdom, also known as the Union Jack, has an aspect ratio of 1:2. It consists of three crosses overlaid on each other:

1. St. George's Cross: a red cross (#CE1126) on a white background (#FFFFFF).
2. St. Andrew's Cross: a white saltire (#FFFFFF) with a blue background (#012169).
3. St. Patrick's Cross: a red saltire (#CE1126) on a white background (#FFFFFF).

Geometric properties and measurements:

• The width of the red St. George's Cross is 1/15th of the flag's height.
• The white fimbriation around the red St. George's Cross is 1/30th of the flag's height.
• The width of the white St. Andrew's Cross is 1/15th of the flag's height.
• The width of the red St. Patrick's Cross is 1/15th of the flag's height.
• The white fimbriation around the red St. Patrick's Cross is 1/30th of the flag's height.

The flag features a counterchanged design in the first and fourth quarters, where the red St. Patrick's Cross lies above the white St. Andrew's Cross, while in the second and third quarters, the white St. Andrew's Cross lies above the red St. Patrick's Cross.