As you read this guide, an important fact you need to keep in mind is that "for pianos, size matters". Simply put, the bigger the piano is, the better it sounds. This is because pianos produce the sound by vibrating strings, which means that the harmonics of a piano are dependent on the length and thickness of the string. If the string get too short, the quality of the sound produced (especially the low frequency sounds) will not be as musically pleasant. Also, a bigger piano has a larger soundboard which can produce a better sound.
Now, we will learn about the two types of acoustic pianos; upright and grand. You will see how they are different. Let's start with upright pianos.