Since I'll likely be here a while, I should probably have some idea of the unwritten rules - if only so that my (what I imagine has to be, to their ears) grating accent is the only thing that makes me stand out. A few that I've guessed at so far-- - No khakis / chinos, no way. I've seen (a lot) more black or grey jeans in the office. Don't really understand why but that one seems pretty universal. - Friday is just another day. People don't seem to dress much more casually on Fri than they do Mon - Thurs. This is for a job in the city and, unfortunately, the answer isn't 'just wear a tie' since tielessness is an important signifier in this line of work.