. Pierpont Morgan was a ruthless financial wizard with superb taste, whose monument--the library designed by McKim, Mead & White in 1906--was a surprisingly restrained product of America's first gilded age. Still more astonishing in that country's latest era of obscene excess, the Morgan has been doubled in size without losing its distinctive personality. The Renzo Piano Building Workshop has wrought its customary magic in weaving together old and new, strengthening the sense of place, and opening up the new central court to views of the street on three sides. Visitors walking into this serene, light-filled atrium, or looking down from two upper-level balconies can savour the sensation of floating within a transparent bubble at the heart of the metropolis

Nearly all museums have a compulsion to expand, to display more of their holdings and find room for new acquisitions, but also to accommodate ever-greater crowds and boost revenue. A happy few, like the Frick, stay small and are cherished for doing so. In contrast, the Museum of Modern Art abandoned its early role as a tightly focused shrine of the avant garde, and turned itself into an overpoweringly vast emporium with all the appeal of a convention centre. By choosing Piano, who cares as much for the sacred (contemplating art) as the profane (socialising, shopping and eating) and manages to keep the two kinds of space distinct, the Morgan avoided that fate.

The institution badly needed more gallery and storage space for its 350 000-item collection of rare books, master drawings, and manuscripts that range from priceless medieval miniatures to musical scores and correspondence from Ernest Hemingway, plus a better performance space for its renowned concerts. It also wanted to appear less intimidating (Morgan's library was a hermetic strong-box, designed to exclude the hoi polloi and natural light) and to develop its role as an art museum.

For the architects, the challenge was to find a footprint on which to build. The library, the 1850s Morgan family brownstone to the north, and the Classical-style annex that J. P.'s son added in 1928 were all listed properties, and the spaces between were cluttered with later additions. The Landmarks Commission would have opposed a tower. The solution was to clear the additions and to go down, blasting out the Manhattan schist to a depth of 18 metres to accommodate three levels of storage vaults, and a steeply raked auditorium. More than 50 per cent of the 13 800sqm complex is now located below ground. Three new pavilions have been inserted between the existing buildings: offices on 37th Street to the north, a 6m cube called the Thaw Gallery to the south, and a three-storey entry pavilion on Madison Avenue that, in its transparency, offers a symbolic welcome mat. New and old structures frame the 15m, glass-roofed courtyard, evoking an Italian piazzetta.