BT site engineering code
A BT site engineering code is a group of letters assigned by BT, or its predecessor the General Post Office, to a physical location which is equipped by the company with unusual amounts or types of telecommunications. Such codes relate to both BT's own buildings and major customer sites. Historically, site codes beginning with the letter Q were government or defence sites. Codes beginning with the letter Y were radio-related sites. The latter category broadened somewhat over time to include many other kinds of customer site.