One of the major technical innovations that helped to create the sound of the modern piano was the use of a strong iron frame. Also called the "plate", the iron frame sits atop the soundboard, and serves as the primary bulwark against the force of string tension. The increased structural integrity of the iron frame allowed the use of thicker, tenser, and more numerous strings. In a modern grand the total string tension can approach 20 tons.
All else being equal, longer pianos with longer strings have better sound and lower inharmonicity of the strings. Inharmonicity is the degree to which the frequencies of overtones (known as partials, partial tones, or harmonics) depart from whole multiples of the fundamental frequency. Pianos with shorter, thicker, and stiffer strings (e.g., baby grands) have more inharmonicity.
