What constitutes a “Major” museum?

What constitutes a “Major” museum? This announcement from artdialy.org gives us one way of looking at it:

Philippe de Montebello – the eighth and longest-serving Director of The Metropolitan Museum of Art in New York – announced in January his plans to retire at the end of the year. To celebrate Mr. de Montebello’s long stint as Director, the curators of the Museum announced plans today for an exhibition of approximately 300 of the more than 84,000 works of art acquired during his 31-year tenure. The project will be a collaboration of the curators currently working in the Museum’s 17 curatorial departments.

The Philippe de Montebello Years: Curators Celebrate Three Decades of Acquisitions will be on view in The Tisch Galleries from October 24, 2008, through February 1, 2009. With so much to choose from, the 300 selected items promise to be significant. It's on my calendar.