A piano’s action is an orchestra of parts working in concert to give the pianist control of the music. Many things must take place between playing a key and hearing a sound from the piano.
M. Steinert & Sons Patrick Elisha explains the "key" differences in the following video.
Let's explore what happens inside the piano when you play. The journey begins as you depress a key. This motion transfers the energy of your finger through a series of levers and rotating parts which culminate in the hammer striking the string, producing a symphony of color and sound.
The action of a grand and upright piano differ in several ways, resulting in a profound impact on a pianist’s perception of control. The so-called ‘double escapement’ grand piano action offers more versatility and control over dynamics while playing. The upright piano offers a compact layout that takes up minimal space in the home but with less control over the music due to its simpler action design.
Let us start with the upright piano action. The strings in this type of piano are perpendicular to the floor. Upon depressing a key, a part of the action called the jack is rotated forward which then sends the hammer towards the string, in a motion similar to someone knocking at a door. This design requires the use of springs to help the hammer and key return back to their resting position. This means that repetition, specifically rapidly repeating notes on the same key, feel differently on an upright piano.
Due to the nature of its simple design, the upright piano requires the pianist to lift the key almost entirely back up to its resting position before playing the note again, thereby limiting its ability to repeat quickly compared to a grand piano.
Today’s modern upright action is the direct descendant of this early single-escapement style action, patented by English piano maker Robert Wornum in the 1810’s.
In the 1820’s, French piano maker Sébastien Érard patented the “double-escapement grand action,” representing a drastic leap forward in keyboard performance. This allowed greater possibilities at the piano and ushered in the romantic era of piano music by the likes of Chopin and Liszt.
Érard’s double-escapement action allows the hammer to re-strike the string without the pianist having to bring the key back up to its full resting position. This means a significant gain in repetition speed and control compared to the upright piano’s single-escapement action. Due to the fact that grand piano strings are parallel to the floor, the grand action relies primarily on gravity to return to rest (as opposed to the upright’s reliance on springs). In a nutshell the grand piano is easier to play and offers more control, the better the touch, the more satisfying the playing experience. This is why the grand piano action is universally preferred over the upright by professional pianists throughout the world.