alt Hacker NewsInstruments which have a non-discrete set of pitches (as well as voices) will tend towards the more harmonious (so to speak) tuning when playing in harmony. You’ll notice this in choirs, for example, where singing a capella, the chords will follow nice integral ratios of frequencies. Fretless string instruments and the trombone are obvious cases of instruments which can do micro-tuning, but it’s worth noting that brass instruments have finger loops on some of the valve loops to allow adjustment of pitch. Micro-tuning of the pitch can also be managed in wind instruments through adjustment of the embouchure so while woodwinds seem like they would be only capable of discrete pitches, there is some ability to adjust the pitch during performance.
On a church gig in the 90s, I encountered an organ which was not tuned in equal temperament so that playing guitar with the organ always sounded out of tune (something I only discovered once Mass began since we had rehearsed with a piano) and I had to switch to bass to be able to play an accompaniment that sounded decent.
Brass instruments have a finger loop on the third valve loop, but it's not primarily for adjusting to just temperament.
Most brass instruments have three valves. The first lowers the pitch by a tone. The second lowers the pitch by a semitone. The third lowers the pitch by a tone and a half. If you need to lower the pitch by two tones, then you press the second and third valves at the same time, and that works fine. However, if you need to lower the pitch by three tones, then you need to press all three valves at the same time. However, that adds the length of all the valve loops together to the total length of the instrument, whereas to lower the pitch by a fixed interval you need to multiply the length of the instrument by a certain amount, and so to truly lower the pitch by three tones you need to add a little bit more length beyond that supplied by pressing all three valves together. That's what the finger loop on the tubing for the third valve is for, so you can slide it out a bit for certain low notes.