The tunings worth knowing — the note of each string from low to high, what the tuning is used for, songs that live in it, and how to get there from standard. Tap a string to hear its pitch.
Standard tuning (E A D G B E) is a compromise — it puts most chords and scales within reach of four fingers. Alternate tunings trade that flexibility for something specific: a heavier low end (Drop D), a chord that rings out on the open strings (the open tunings, built for slide), or a droning, modal ambiguity with no third (DADGAD).
Tuning down a step or two is safe. Bigger drops lose string tension and can buzz — a heavier string gauge fixes it. Use the tuner to get there accurately, and check where the notes now fall with the Fretboard Trainer.
Once you are in a new tuning, the chord shapes change. The Chord Library shows every chord's notes so you can work out the new shape, and the Cloud Atelier guitar guides cover playing in alternate tunings.