The standard tuning of a six-string guitar, from lowest to highest string, is E-A-D-G-B-E.
The lowest string is the thickest and is tuned to E2; the highest string is the thinnest and is tuned to E4. Most adjacent strings are separated by a perfect fourth, but the G-to-B interval is a major third. That irregular interval helps make common chord shapes practical across the neck.
This tuning is used on acoustic guitars, electric guitars, and many classical guitars. It supports familiar open chords and scales while providing a useful balance between low bass notes and high melodic notes.
Many alternative tunings exist. Drop D lowers only the sixth string from E to D, while open tunings create a chord when the strings are played open. These alternatives are important stylistically, but E-A-D-G-B-E remains the conventional reference tuning.