is a way to train technique and strength by going through the same motions as rowing, with resistance(usually a large tank of water). Indoor rowing is helpful when there are no rowable bodies of water near by, or weather conditions don't permit rowing. A rowing tank is an indoor facility which attempts to mimic the conditions rowers face on open water. Rowing tanks are used primarily for off-season rowing, muscle-specific conditioning and technique training, or simply when bad weather prevents open-water training. Ergometer rowing machines (colloquially ergs or ergo) simulate the rowing action and provide a means of training on land when waterborne training is restricted, and of measuring rowing fitness. Ergometers do not simulate the lateral balance challenges, the exact resistance of water, or the exact motions of true rowing including the sweep of the oar handles. For that reason ergometer scores are generally not used as the sole selection criterion for crews (colloquially "ergs don't float"), and technique training is limited to the basic body position and movements. However, this action can still allow a workout comparable to those experienced on the water. Indoor rowing has become popular as a sport in its own right with numerous indoor competitions (and the annual World Championship CRASH-B Sprints in Boston) during the winter off-season. Race formats There are several formats for rowing races, often called "regattas". The two most common are side by side and head races. Side by side Most races that are held in the spring and summer feature side-by-side, or sprint racing; all the boats start at the same time from a stationary position, and the winner is the boat that crosses the finish line first. The number of boats in a race typically varies between two (which is sometimes referred to as a dual race) to eight, but any number of boats can start together if the course is wide enough. The standard length races for the Olympics and the World Rowing Championships is long. In the United States, some scholastic (high school) races are , while many youth races are the standard 2 kilometres. Masters rowers (rowers older than 27) often race 1,000 m. However the race distance can and does vary from dashes or sprints, which may be long, to longer dual races like the Boat Race. Two traditional non-standard distance shell races are the annual Boat Race between Oxford and Cambridge and the