Compare the tuning pins in these two pianos. They are around the same age and both are school pianos. I advised the music teacher that the one on top was very concerning because of the amount of corrosion. Rust on strings and tuning pins is always a red flag! Having another piano around of similar age and design helped demonstrate the problem in this case. Often piano owners don’t notice rust or corrosion in their piano because it looks “normal” to them or they don’t ever look inside their instrument. Sometimes you can learn a lot just by a quick look!
Besides the rust, there is another problem with this picture. The wire coiled around each tuning pin is flush against the plate. This happens after several decades of tuning and settling, and is largely unavoidable. Once the coil is touching the plate, tightening the tuning pins is difficult and the piano’s tuning stability can suffer. Eventually the tuning pins have to be replaced with a slightly larger size, at which point it usually is most sensible to simply re-string the entire instrument because of the labor involved. Fortunately, this piano was about to get re-strung anyway!
A standard piano has 88 keys, but well over 200 strings. Typically the lowest octave or so consists of single-string notes, or monochords. The next section, usually ending somewhere in the octave below middle C, consists of two-string notes: bichords. All monochords and bichords are wound strings, meaning they have a steel core but have one or two layers of copper winding on top which increase the size of the string and yield a lower pitch. The rest of the piano consists of three plain-wire strings for each note.And that is part of why it takes a while to tune a piano. Not to mention re-stringing one!