Originally (on early writers dating back to the late 19th century) the "bar" was literally a metal bar running across the full width of the keyboard (or even wider, and even surrounding it) that triggered the carriage advance without also firing any of the typebars towards the platen.
Some early typewriter and particularly computer keyboards used a different method of inserting spaces, typically a smaller, less distinct "space" key which was also often set in a less central position, e.g. the Hansen Writing Ball, Hammond typewriters or the Sinclair ZX Spectrum and Jupiter Ace ranges.