In grand pianos, the frame and strings are horizontal, with the strings extending away from the keyboard. The action lies beneath the strings, and uses gravity as its means of return to a state of rest.
There are many sizes of grand piano. A rough generalization distinguishes the concert grand (between 2.2 and 3 metres long, about 7–10 feet) from the parlor grand or boudoir grand (1.7 to 2.2 metres long, about 6–7 feet) and the smaller baby grand (around 1.5 metres (5 feet)).
All else being equal, longer pianos with longer strings have larger, richer sound and lower inharmonicity of the strings. Inharmonicity is the degree to which the frequencies of overtones (known as partials or harmonics) sound sharp relative to whole multiples of the fundamental frequency. This results from the piano's considerable string stiffness; as a struck string decays its harmonics vibrate, not from their termination, but from a point very slightly toward the center (or more flexible part) of the string. The higher the partial, the further sharp it runs. Pianos with shorter and thicker string (i.e. small pianos with short string scales) have more inharmonicity. The greater the inharmonicity, the more the ear perceives it as harshness of tone.
Inharmonicity requires that octaves be stretched, or tuned to a lower octave's corresponding sharp overtone rather than to a theoretically correct octave. If octaves are not stretched, single octaves sound in tune, but double—and notably triple—octaves are unacceptably narrow. Stretching a small piano's octaves to match its inherent inharmonicity level creates an imbalance among all the instrument's intervallic relationships, not just its octaves. In a concert grand, however, the octave "stretch" retains harmonic balance, even when aligning treble notes to a harmonic produced from three octaves below. This lets close and widespread octaves sound pure, and produces virtually beatless perfect fifths. This gives the concert grand a brilliant, singing and sustaining tone quality—one of the principal reasons that full-size grands are used in the concert hall. Smaller grands satisfy the space and cost needs of domestic use.