In the centre of the ceiling of the Gothic Library at Stowe is an amazing work of heraldry: The Stowe Armorial.
The Library was commissioned by George Nugent-Temple-Grenville, 1st Marquess of Buckingham (often called Nugent Buckingham). It was built to a design by Sir John Soane between 1805 and 1807. The armorial is a 1.4m diameter heraldic painting of the 719 quarterings of the Temple, Nugent, Brydges, Chandos and Grenville families, including ten variations of the English Royal arms, the arms of Spencer, De Clare, Valence, Mowbray, Mortimer and De Grey. The painting is signed and dated P. Sonard 1806 (see Stowe House, Michael Bevington 2002).
Somewhere, sometime in a book and certainly on the Internet, this fascinating work has been renamed the Grenville Diptych. That, to put it colloquially, it ain’t. The OED tells us that a diptych is “an altar-piece or other painting composed of two leaves which close like a book.” (http://www.oed.com/view/Entry/53277). The Stowe Armorial does not have two leaves, neither can an immovable object in a ceiling be folded.
This myth is widespread and on eBay and on Amazon you can buy prints of the “Grenville Diptych”. They are lovely images but they are not images a diptych.