The Rhodes piano was invented in the 1940s by Harold Rhodes, and its principles are derived from both the celesta and the electric guitar. The action is similar to that of a conventional piano, but whereas in a conventional piano each key causes a felt-covered hammers to strike a set of strings, in a Rhodes piano they strike a tuning fork-like construction to sound the note.
The tuning forks themselves are "unbalanced" or asymmetrical: one arm consists of a short, stiff metal rod (essentially a stiff wire) called a "tine" which is struck by the hammer, and the other arm is a tuned resonator resembling a piece of metal bar stock, sized to sound the appropriate note. The actual sounded note is too soft to be practical, so each tine vibrates in front of an electric-guitar-style magnetic pickup. The pickups' output is fed to an amplifier which can be adjusted to produce the desired volume.
The sound produced has a bell-like character not unlike a celesta or glockenspiel. Because the instrument produces sound electrically, the signal can be processed to yield many different timbral colors. Often the signal is processed through a "delay" or "chorus" effects box, which adds a distinctive vibrato similar to a vibraphone; it is this "rounded" or chiming sound that is most typically called a classic Rhodes sound. When notes are played forcefully, the sound becomes less sweet, as nonlinear distortion creates a characteristic "growling" or "snarling" overload -- skilled players can contrast the sweet and rough sounds to create an extremely expressive perfomance.
The Rhodes was particularly popular during the 1960s and 1970s, and many of its signature songs date from this period: "Just the Way You Are" by Billy Joel, "Still Crazy After All These Years" by Paul Simon, "Dark Star" by Crosby, Stills and Nash, or the theme from "Taxi" by Bob James.
The Fender Guitar Company bought the Rhodes company in the 1950s, and produced the instruments for many years, in conjunction with Fender-designed amplifiers. The instrument is thus often termed a "Fender Rhodes" though purists prefer simply "Rhodes." The actual instruments are more rare in latter days: they are fragile, heavy, and tedious to tune. All modern synthesizers contain built-in "electric piano" patches that approximate the signature Rhodes sound with considerably more convenience.