This is not, however, an exhibition about the stylistic development of portraiture but about the different ways that portraits were used. "Through painting, the faces of the dead go on living for a very long time," noted the architect and theorist Leon Battista Alberti, but patrons quickly realised that portraits could also be exchanged as diplomatic gifts, display the sitter's social standing, give prospective husbands an idea of the beauty - or otherwise - of their bride-to-be and keep friends and lovers close at hand. They could amuse, too, as is evidenced by a rare appearance of a work by Giuseppe Arcimboldo. His c.1590 portrait of the Emperor Rudolph II as Vertumnus, the god of the seasons, must surely be the most bizarre royal portrait ever painted. The emperor is composed of vegetables, fruit and flowers - a pear for a nose, a corncob for an ear, a marrow for a forehead. How the scientifically minded Rudolph must have laughed, though not perhaps as much as his courtiers.

This exhibition is remarkable for both the quality of the exhibits and their variety. As in any good social gathering, the stately and the beautiful mingle with the lively and the ugly, their massed gazes offering a blink of recognition across the centuries.

Many of the sitters in the National Gallery were inhabitants of the world on display at the Queen's Gallery at Buckingham Palace: Bruegel to Rubens: Masters of Flemish Painting (from October 17). The exhibition covers the period 1500-1665, the golden age of Flemish art but also a time of bitter civil strife as the inhabitants of the Low Countries sought to free themselves from their Spanish overlords.

Unlike the gallery's The Art of Italy exhibition of last year, this is not a show full of surprises; most of the artists - the likes of Marten van Heemskerk, Crispin van den Broeck, Frans Wouters - justify their relative anonymity. There are, however, some spectacular pictures by Pieter Bruegel the Elder, Rubens and Van Dyck that show once again just what riches lurk in the Royal Collection.