] ] (If she had not had a title of her own, she would have become Lady Lewis de Burgh, like being Mrs. John Jones.)

] A minor correction - all correct except the above...

] Sir Lewis de Burgh and Lady de Burgh, not Lady Lewis de Burgh.

Sorry, Lady Catherine de Burgh would have been Lady Catherine de Burgh even if she had married Mr. de Burgh not Sir Lewis. Remember Lady Marjorie Bellamy in "Upstairs, Downstairs"? She was married to plain Mr. Richard Bellamy (though he became Lord Bellamy after her death).

Lady (plus her husband's given name) is the wife a the younger son of a peer, like Lady Peter Wimsey or Lady Randolph Churchill (Winston's mother). Both were the wives of younger sons of Dukes.