General Notes

Sir William Douglas, 3rd of Douglas; born 1240; died 1274, leaving [Sir William], with an elder son (Hugh, dsp by 1289) and a daughter (Willelma, married 1st William, Stn Chief of the Galbraiths; married 2nd Gilbert of Butternock (in right of his wife) and with him was probably ancestor of the Dukes of Abercorn.)  [Burke's Peerage]

Note on birthdate of William.  Burke's does state 1240, but also states that he was born before Andrew.  Many Others indicate a much earlier birthdate, so I will use 1214 taking 1240 for some kind of misprint, maybe they meant his son William who was born c1240.

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Copied from "Douglas Family" by Mark Freeman, freepages.genalogy.rootsweb.com/~markfreeman/douglas.html:
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Notes for Sir William of Douglas:  "Longlegs"

First son of Archibald de Douglas and father of William "le Hardi", William "Longlegs" was born circa 1200 and died in 1276. He was reported to be "of tall and goodly stature" and so by acquired his pseudonym. William married the sister of the Earl of Carrick, who in turn was the grandfather of Robert The Bruce. William's brother, Andrew de Douglas was the sire of the Morton line of Douglases.

"William, the elder, inherited the estate of his father. William acquired additional lands to the family inheritance; and, by this means, becoming a tenant in chief of the crown, was considered as ranking among the barons, or, as they were then called, magnates of Scotland."

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http://www.scotclans.org/history/other/douglas_james.htm

Sir William was a witness to a charter in 1240, and, along with Sir Andrew of Dufglas, to another charter in 1248.