Solfeggio is a great resource for tuning and developing relative pitch. By practicing this consistently, you will often recognize the distance between one note and another or even chord progressions without needing the instrument.

I never knew that there is a proper term for those vocal exercises! Today I learn

cryotosensei

The most I do is solfeggio when I'm trying to learn a song. As I have reasonable relative pitch, I use this device to discover the chords