http://en.wikipedia.org/wiki/William_Montacute,_1st_Earl_of_Salisbury

http://en.wikipedia.org/wiki/Roger_Mortimer,_2nd_Earl_of_March

Roger married Philippa Montacute, daughter of William Montacute, 1st Earl of Salisbury

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Sir William de Montagu [g], Earl of Salisbury, b abt 1302, Cassington, Oxfordshire, England, d 30 Jan 1343/44. He md Katherine de Grandison 1327, Oxfordshire, England, daughter of Sir William de Grandison and Sibyl de Tregoz.

Children of William de Montagu and Katherine de Grandison were:

Philippa de Montagu b abt 1332, Salisbury, Wiltshire, England, d abt 1381, Berkshire, England. She md Sir Roger de Mortimer, Earl of March, abt 1345, son of Sir Edmund de Mortimer, Earl of March, and Elizabeth de Badlesmere.

Sibyl de Montagu b abt 1334. She md Edmund Fitz Alan bef Jul 1349, son of Sir Richard Fitz Alan, Earl of Arundel, and Isabel le Despenser.