I've been learning more recently about how most western music these days evolved from pure tuning to well-tempered tuning. It's fascinating that in theory musical intervals "should" be spaced out mathematically according to certain ratios to sound truly "in tune", but it's almost impossible to physically reproduce that exact tuning with an instrument - thus in the 1700s well-tempered tuning was devised to approximate "being in tune" on an instrument - across all keys, not just one fundamental key.
Some interesting excerpts from Gregsguitar.com on how that affects guitar-players. Apparently with well-tempered tuning 4ths and 5ths and octaves are in tune, but 3rds are not. Jeez, no wonder I always am off when I tune my B string by ear haha.
"All Possible Keys Tempered tuning revolutionized music during the period of J.S. Bach. Bach was one of the first to write music for a 'well-tempered clavier.' Now it was possible to play chords and scales in any of 12 keys. Today, the key of 'Gb' plays in tune just as well as in 'C' or any other key. For the first time, it was possible to change keys in the middle of a piece, and to expand harmony to include all of the notes that were excluded before. This is when European keyboard and orchestral music first began to use many of the musical forms we recognize today.
Sometimes when you're recording a guitar part where a 3rd or a 6th is prominent, it's good tojust-tune that interval to decrease the enharmonicity. This usually means lowering the 3rd or 6th relative to the tonic ('one-note').
Built-In Dissonance Tempered tuning contains built-in dissonance, also known as 'enharmonicity.' This is the reason you can't tune with third or sixth invervals. Thirds and sixths which sounded so pure in just-tuning were 'sacrificed' so that octaves, fourths and fifths are always in tune in any of the 12 possible keys, and in all octaves simultaneously."
