In the context of British Inland Waterways, "narrow boat" refers to the original working boats built in the 18th, 19th, and 20th centuries for carrying goods on the narrow canals (where locks and bridge holes would have a minimum width of 7 feet or 2.1 metres). Some locks on the Shropshire Union are even smaller. The term is extended to modern narrowboats used for recreation and more and more as homes, whose design is an interpretation of the old boats for modern purposes and modern materials. The cargo-carrying steam narrow boat was always something of a rarity because the space taken up by the smallest steam plant so reduced the carrying capacity that the boat became uneconomic unless it towed at least one other craft. Such operation was practical only on long, lock-free stretches of a canal, or on a wide canal where both boats could use the same lock, thus the steam narrow boat was largely confined to the wide Grand Junction Canal and its narrow connections to Birmingham and Leicester.