The Amsterdam canal system is the result of conscious city planning. In the early 17th century, when immigration was at a peak, a comprehensive plan was developed that was based on four concentric half-circles of canals with their ends emerging at the IJ bay. Known as the Grachtengordel, three of the canals were mostly for residential development: the Herengracht (where "Heren" refers to ''Heren Regeerders van de stad Amsterdam'', ruling lords of Amsterdam, whilst ''gracht'' means canal, so that the name can be roughly translated as "Canal of the Lords"), Keizersgracht (Emperor's Canal) and Prinsengracht (Prince's Canal). The fourth and outermost canal is the Singelgracht, which is often not mentioned on maps because it is a collective name for all canals in the outer ring. The Singelgracht should not be confused with the oldest and innermost canal, the Singel.