For Grimshaw, it is Wright's steady growth as a craftsman-architect that is important: the evolution from his early experiments continually tinkering with his modest family house in Oak Park, Chicago, to a work of mature genius such as the Johnson Wax building in Racine, Illinois. "It takes a hell of a long time," says Grimshaw. "Look at Frank Gehry, or Fumihiko Maki...you learn, you get better." Equally he admires the steadily evolving oeuvre of Renzo Piano, whom he regards as a master of materials. Hans Scharoun and Alvar Aalto, in contrast, he finds intriguing for other reasons; architects who were much taken with materials and texture and the sequence of flowing spaces, but who were far less interested in structure (and in Scharoun's case, detail) than he is. The fact that he mentions these names is interesting: one thinks of the Aalto-esque sinusoidal roof inside the Vitra building way back in 1983, or the way the practice increasingly tends to use flowing and curving forms rather than right-angles, both inside and outside the later buildings. Like professors of anatomy, the senior NGP architects believe that you have to have a thorough understanding of the bone and sinew beneath the skin in order to arrive at the eventual form.

Determined that this accumulated learning and reputation should not be dissipated, Grimshaw as chairman has overseen a restructuring of the practice so that eventually half of the shares in the business will be owned by the directors - Harriss, Nash, Sidor and Whalley - and half by an employee benefit trust. This arrangement is based on the model of the engineering partnership Ove Arup, with which NGP has worked on many projects over the years: the idea of a common cause, commonly owned. Thus, it is impossible for anyone else to buy the firm, and equally impossible for it to be broken up. Nor can it be floated on the Stock Market, something that would create an immediate conflict of interest between clients on the one hand and shareholders on the other. Grimshaw is happy with the fairness and durability of the new structure. "I thought long and hard about this. It's the idea of quality, of having bright people coming up from the bottom, and subscribing to a common philosophy of excellence," he says. "Where you can actually turn down work that is not right."

A steady evolution, then, explains the consistency of approach that Nicholas Grimshaw and Partners has shown over the years. For this reason there is a feeling, in the world of architecture and outside, that the practice is somehow apart, operates according to its own system. Labels - beyond the catch-all one of "progressive", tend not to stick easily. It has never deviated from its course to nod to fashion - and was consequently seen by some as being out of step in the early post-modern era. This never ruffled Grimshaw or his colleagues, who anyway tended to keep a low profile, preferring to run a practice known for clarity of thought rather than short-term glamour. They tend to be private people, which has a lot to do with it.