The luxury listings of Boston, however scrumptious, can certainly run together. That is why when one actually gives us pause not only for its grandiosity but for its perspicacity, we take especial note. Herewith, then: the just-listed 6,186-square-foot townhouse at 49 Mount Vernon Street in Beacon Hill, dating from the presidency of the Duke of Braintree himself. The seller bought the townhouse in 1964, and added three rental units (one of which, the listing assures us, could be worked back into the main house). Otherwise, the townhouse has nine bedrooms, five bathrooms and 10 fireplaces as well as a large garden and—here was the perspicacity—an "ell." Us neither: It's a wing of a building that is at right angles to the main part of the building. All yours for $4,350,000.