Water closets (WC, for short) are rooms (usually small, but larger than closets meant for clothing or storage) in homes or other buildings which are furnished with pipes and fixtures for the transport of water into and out of the building, for purposes of drinking, bathing, and sewage (a WC typically includes a sink, a bathtub, and a toilet; it's known that on Earth, the term "water closet" has sometimes been applied to the toilet itself, as well as the room that houses it). However, plumbing can also be installed in other rooms, such as a kitchen or laundry room.
These systems are designed by engineers, but installed and maintained by plumbers. While similar systems had been used for several centuries for irrigation (and some would also include water wells under the heading of plumbing) prior to the introduction of indoor plumbing, it wasn't until the invention of pumps and other components (by Lyle) that indoor plumbing for personal use became practical. The first water closets were developed in Monab sometime in the 890s, and became one aspect of The Plan conceived by Bishop Kizin in LY 890. Water closets were first introduced outside of Monab in 897, in the villages of the Northern Alliance, where Monabite engineers and plumbers trained others in the production, installation, and maintenance of the necessary systems.
Engineers and plumbers of the Northern Alliance began travelling to other villages across The Land in 901, installing plumbing and teaching people in the villages they visited how to maintain the equipment. The rooms represented a dramatic advantage over outhouses and bath houses, making life in general more convenient, expedient, sanitary, and just more... pleasant. Naturally, they quickly became popular, though the expense and trouble of installing them means they are even now not quite ubiquitous.
One innovation of northern engineers was to add stoves to the water closets, for heating the water that was piped into bath tubs. This is particularly convenient in their colder months, a problem not shared with southern villages, though even in such villages, there is some demand for water-heating stoves. It has even been suggested that water for purposes other than bathing- such as washing hands, clothes, or dishes- would be well served by being heated. This is beginning to be done in some businesses, though it will likely take longer to reach private residences.