It is at the Victoria and Albert Museum. It would have to be, really. The V&A has always collected real bits of real old buildings - and for that matter full-size plaster casts of classical ruins - but these full-size fragments are presented less as architecture, more as sculpture. There are exceptions - the complete interior of a Frank Lloyd Wright office from the late 1930s is a great thing to stumble across, for instance - but on the whole architecture has not been able to break through the pervasive sense that the museum is the nation's attic-full of interesting and valuable junk. Rather than being the standard-bearer of arts and design it is meant to be.

But there is another thing. There are two world-class collections of architectural drawings, models and objects in London. One is the V&A's. The other belongs to the Royal Institute of British Architects. There is a lot to be said for the logic of combining these two collections - neither of which has to date got much of a public airing - in a purpose-designed gallery. There is so much to be said for it, in fact, that this is exactly what has finally happened. The new gallery, with its associated study room and archives, is a £10m joint venture. And it is rather good. From Palladio to Zaha Hadid via Richard Rogers and Norman Foster, this is a fast 'n' friendly romp through the history of architecture.

"No other institution in the world is doing what we are doing here," says Charles Hind, curator of the RIBA's collection, as we tour the nearly-completed gallery with Michael Snodin, his opposite number at the V&A. They should know: they have travelled the world's museums to see how it is done elsewhere, and found that, by and large, it wasn't. Oh yes, there are architecture galleries in places like Rotterdam, Oslo, or Stockholm, but those tend to tell a national, or a modern-day story. Here, they are telling an international one across history. And as always, the trick is to grab people's attention without dumbing down.

"This is aimed at the general public," says Snodin. "We're not assuming they know anything about architecture. Our aim is to unpack architecture so people understand why buildings are the way they are." Interesting that the Heritage Lottery Fund should have part-funded this, while refusing to countenance the museum's now-axed "Spiral" extension for contemporary design by Daniel Libeskind.

Architect Gareth Hoskins has taken one of the big galleries high in the imposing frontage to the V&A, as originally built - in ramblingly eccentric style - by Sir Aston Webb at the start of the 20th century. Previously the new gallery was one of the museum's half-forgotten spaces, crammed with a study collection of French ceramics. Now the ceramics have been moved elsewhere, a reorganization of the museum has put it on a better circulation loop that leads on to the Glass Gallery (soon to be revamped) and back through a new sculpture gallery. The approach staircase has been opened up and restored to its Aston Webb grandeur. And Hoskins has designed his space to catch your attention and draw you in, rather than glance at and pass by.

A giant drawing - a cutaway bird's eye perspective of St. Paul's Cathedral, which took an undeniably obsessive architect years to do in the 1920s - signals part of what is to come from the lobby outside. The present day is covered by an equally large montage of frequently-changed images of today's architecture. Once in the gallery proper, you follow three threads - the style of buildings, how they work, and how architects design them - arranged down the length of the room. At the end, you take a left into a temporary exhibition space, carved out of what was previously a storage area. "Great Buildings" is the catch-all opening show there. More individualistic shows will follow.