Complex sentence: John Graham, Earl of Menteith (d.28 Feb 1347) became Earl of Menteith by courtesy of his wife, Mary Menteith, Countess of Menteith in her own right, daughter of Alan Menteith, 7th Earl of Menteith.
Split into simpler sentences: John Graham, Earl of Menteith (d.28 Feb 1347) was a Scottish nobleman. Graham became Earl of Menteith by courtesy of his wife, Mary Menteith, Countess of Menteith in her own right, daughter of Alan Menteith, 7th Earl of Menteith.