All peers above the rank of baron are officially addressed by the Crown as cousin.
A viscount or earl is “Our right trusty and well-beloved cousin.”
A marquis is “Our right trusty and entirely-beloved cousin.”
A duke is “Our right trusty and right-entirely-beloved cousin.”
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Entry taken from Dictionary of Phrase and Fable, edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.
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