A grand piano is identified by it's action, not just the shape of the instrument. Simply put, if the action of the piano comes from "underneath" the strings, it is a grand piano. If the action of the piano is in front of the strings, it is a vertical or "upright" piano.

The two basic advantages of grand pianos strictly from a performance perspective is the action, which tends to be smoother and allows for a quicker repetition and the fact that the soundboard is exposed on both the top and bottom. The soundboard of an upright piano is exposed on the back only and typically, once in the home, the piano is placed against a wall reducing the exposure further.

The term "baby grand" and "grand" are often used incorrectly. A baby grand piano is any grand piano that measures less than 6' in length. Any grand piano longer than 6' is considered a grand piano - nothing more. There are baby grand pianos that are far superior in quality to actual grand pianos and of course, vice versa. The length or "size" of a grand piano is the distance from the keyslip (the rail located just in front of the white keys) to the farthest point away from you, measured with the piano lid closed.