A state room in a large European mansion is usually one of a suite of very grand rooms which were designed to impress. The term was most widely used in the 17th and 18th centuries. They were the most lavishly decorated in the house and contained the finest works of art. State rooms are usually only found in the houses of the upper echelons of the aristocracy, those who were likely to entertain a head of state. They were generally to accommodate and entertain distinguished guests, especially a monarch and or a royal consort, or other high ranking aristocrats and state officials, hence the name. In their original form a set of state rooms made up a state apartment which always included a bedroom.

In England in particular state rooms in country houses were seldom used. The owner of the house and his family actually lived in the "second best" apartments in the house. There was usually an odd number of state rooms for the following reason: At the centre of the facade, the largest and most lavish room, (for example at Wilton House the famed Double Cube Room), or as at Blenheim Palace (right) this was a gathering place for the court of the honoured guest. Leading symmetrically from the centre room on either side were often one or two suites of smaller, but still very grand state rooms, often in enfilade, for the sole use of the occupant of the final room at each end of the facade - the state bedroom. Unlike the main reception rooms of later houses, state apartments were not freely open to all the guests in the house. Admittance to the state apartment was a privilege, and the further one penetrated (there were many variations, but an apartment might include for example an anteroom; withdrawing room; bedroom; dressing room; and closet) the greater the honour.