Pick up a guitar in standard tuning and the thickest string, the one nearest your thumb, sounds E2. It is the lowest open string on the instrument, the deep foundation the other five are built above, and for most players it is the first note their hand ever settles on. Its identity is exact: E2 is MIDI note 40, ringing at 82.407 Hz, low and round without dropping into the murk of the true bass register. Standard guitar tuning runs E-A-D-G-B-E from the lowest string to the highest, and this E anchors the bottom of that sequence, the point from which the whole tuning is reckoned.

Learn where it sits and the rest of the fretboard begins to make sense. What makes E2 so central is not just that it is lowest but that it is a starting point. Fret and pluck your way up from here, moving along the neck and across the strings, and you cover most of what a guitar actually plays. Countless riffs begin on this open string, countless chords use it as their root, and the low E gives rock and folk alike much of their weight.

It is the string players tune first and check most, the reference the ear returns to. There is a reason guitarists talk about the low E with a certain affection. It is the deepest voice the instrument has in ordinary tuning, the note that grounds a chord and drives a riff, sitting at a comfortable 82.407 Hz where it is unmistakably low yet still clearly pitched. From this one open string, the guitar's whole range opens upward, and almost everything the instrument can say starts somewhere near it.