The open strings on a standard guitar are tuned to the following notes in various tunings:

1. Standard Tuning: E, A, D, G, B, E

2. Drop D Tuning: D, A, D, G, B, E

3. Open G Tuning: D, G, D, G, B, D

4. Open D Tuning: D, A, D, F, A, D

5. Open A Tuning: E, A, D, E, A, E

These tunings are popular among guitarists due to their distinct sounds and ease of playing certain chords and scales. The variations in tuning allow for a wider range of musical possibilities and can be used to achieve different tonal qualities in various genres.

The reason for these tunings lies in the physics of string tension and the frequency of the notes. When a string is plucked, it vibrates at a specific frequency that corresponds to a particular note. The tension of the string, the length of the string, and the mass of the string all affect the frequency of the note. By adjusting the tension and length of the strings, guitarists can achieve different notes.

For example, in standard tuning, the open strings are tuned to the notes E, A, D, G, B, and E. This tuning is based on the harmonic series, which is a sequence of notes that sound good together. The open strings are tuned to the first, second, third, fourth, fifth, and sixth harmonics of the lowest open string, which is E. This tuning allows for easy playing of many common chords and scales.

In drop D tuning, the lowest string is tuned down to D, which is a fifth lower than the lowest open string in standard tuning. This creates a deeper, more resonant sound and allows for easier playing of certain riffs and power chords. Open G tuning, on the other hand, is tuned to the notes D, G, D, G, B, and D, which is a fifth lower than standard tuning. This tuning is popular for fingerpicking and slide guitar, as it allows for a more open, resonant sound.

The choice of tuning depends on the desired sound and the specific musical style. For instance, open G tuning is often used in folk and country music, while drop D tuning is popular in rock and metal. The data supporting these tunings comes from the physics of sound and the historical use of these tunings in various musical genres.