It was in this period that the national flag emerged as a common symbol. The image of St. Andrew martyred bound to an X-shaped cross first appeared in the Kingdom of Scotland during the reign of William I and was again depicted on seals used during the late 13th century; including on one particular example used by the Guardians of Scotland, dated 1286. Use of a simplified symbol associated with Saint Andrew, the saltire, has its origins in the late 14th century; the Parliament of Scotland decreed in 1385 that Scottish soldiers wear a white Saint Andrew's Cross on their person, both in front and behind, for the purpose of identification. Use of a blue background for the Saint Andrew's Cross is said to date from at least the 15th century. The earliest reference to the Saint Andrew's Cross as a flag is to be found in the "Vienna Book of Hours", circa 1503.