In an upright piano, the strings are strung vertically to make the instrument more compact, which allows the piano to be played in a limited space. In contrast, the grand piano retains the shape of the original pianoforte in which the strings are strung horizontally, and has a greater potential for expression.
Differences in the action:
There is a significant difference between the action mechanisms of a grand piano and an upright piano.
Grand piano
Horizontal. Because the hammers return to their rest position under their own weight, repetition, in which the notes are repeated quickly such as when playing trills, is smooth (key repetition as fast as about 14 times per second is possible).
Upright piano Vertical.
Hammers rely on springs to return to the at-rest posit ion, and so there is a limit to fast repetition such as is used when playing trills (key repetition as fast as about seven times per second is possible).
Differences in the pedals
The pedals have different functions in a grand piano and an upright piano.
Grand piano
Shift pedal (left pedal): Also called the soft pedal or una corda pedal. Shifts the entire action assembly to the right, changing not only the sound volume, but also making minute changes to the tone. Sostenuto pedal (middle pedal): Keeps the dampers raised and away from the strings of any keys played just before depressing the pedal. This makes it possible to sustain selected notes. Sustain pedal (right pedal): Also called the damper pedal. Dampers remain lifted even if the fingers are taken off the keys, sustaining all played notes.
Upright piano
Soft pedal (left pedal): When this pedal is pressed, all of the hammers are moved closer to the strings, reducing the sound volume. Muffler pedal (middle pedal): Also called the practice pedal. A thin piece of felt is dropped between the hammers and strings, greatly muting the sound. Sustain pedal (right pedal): Also called the damper pedal. Dampers remain lifted even if the fingers are taken off the keys, sustaining all played notes.