There are two main types of upright piano string arrangement.
Here we will explore the differences between the two, including the reasons one is preferred over the other.
The first variety we will look at is the "straight-strung" arrangement.
The most noticeable characteristic of a straight-strung piano (sometimes referred to as "vertical-strung") is that the strings run parallel and are vertically or obliquely arranged. As photographed above, the strings run from the top of the piano, down towards the bottom in an almost straight direction.
The second variety we will look at is the "over-strung" arrangement.
The most striking and obvious difference of an over-strung piano (sometimes referred to as "cross-strung") is that the strings run diagonally. As photographed above, the strings are positioned in such a fashion that the bass strings cross over the tenor strings in an 'X' style.
Why are over-strung pianos preferred?
Over-stringing was first used around the 1850s and allows for longer strings, a better position of the bridge and calls for a much more substantial frame design.
Rather than running from top to bottom like a straight-strung, the bass strings in an over-strung run almost corner to corner (top left to bottom right), allowing for longer strings; which deliver a richer tone quality and volume. The bridge transmits vibrations from the strings into the soundboard, and is more efficient the further it is from the edge. In a straight-strung, the 'speaking length' of the strings end near the bottom of the piano, so the bridge needs to be at the bottom too. Whereas the over-strung design allows the bridge to be positioned further away, increasing the efficiency and improving tone quality and volume.
The cast iron frame in an over-strung piano is a much more substantial design, compared to the simpler frame of a straight-strung piano. To support the string arrangement, the over-strung frame contains more braces, adding more strength and support thus facilitating better tuning stability.
With a few notable exceptions (e.g C.Bechstein Model 10), straight-strung pianos are inferior to modern over-strung instruments, which is why they are no longer made.
Photographed below is the overstrung frame from a Knight K10