The Scottish national flag is a white-on-blue Saltire (i.e. a diagonal cross on a coloured background) and it derives from the shape of the cross on which St Andrew was crucified.
St Andrew was brother of St Peter, the first Pope, and according to tradition he was bound (not nailed) to a Latin cross of the kind on which Jesus is said to have been crucified, yet in the form called “crux decussata” (X-shaped cross, or "Saltire"), supposedly at his own request, as he deemed himself unworthy to be crucified on the same type of cross as Jesus had been.
November 30th is his feast day.
The legend surrounding Scotland's association with the Saint Andrew's Cross dates from a 9th century battle where Óengus II , King of Alba, led a combined force of Picts and Scots to victory over the Angles and Saxons led by Æthelstan. The legend states that whilst engaged in prayer on the eve of battle, Óengus vowed that if granted victory he would appoint Saint Andrew as the Patron Saint of Scotland. On the morning of battle white clouds forming an X shape in the sky were said to have appeared. Óengus and his combined force, emboldened by this apparent divine intervention, were victorious despite being inferior in terms of numbers.
Having interpreted the cloud phenomenon as representing the “crux decussata” upon which Saint Andrew was crucified, Óengus honoured his pre-battle pledge and duly appointed Saint Andrew as the Patron Saint of Scotland. The white saltire set against a celestial blue background is said to have been adopted as the design of the flag of Scotland on the basis of this legend.
