Country house opera is opera performed at a country house, whether in the house itself, in the grounds, or in an adapted or purpose built theatre on the grounds. It is a feature in particular of the English summer. Country house opera is usually initiated by private enthusiasts, and relies almost exclusively upon private funding. This distinguishes it from the UK's state subsidised opera companies which, like most opera houses around the world, are based in city centres. Black tie is often de rigueur, in contrast with the Royal Opera House in London and other city operas where casual clothing is often acceptable.