Guitar tunings that require a capo are those where the standard tuning (EADGBE) is altered to accommodate a different key or to achieve a desired sound. Capos are used to change the pitch of strings without retuning the entire guitar, allowing musicians to play in various keys while maintaining the same fingerings. Common tunings that often necessitate the use of a capo include open tunings, drop tunings, and alternate tunings like DADGAD or C6.

The need for a capo arises from the fact that certain tunings are designed to produce a unique sound or to facilitate easier playing in a particular key. For instance, open tunings, where the guitar is tuned to a chord shape without fretting any strings, often require a capo to shift the chord shape to a more comfortable key. This is particularly true for tunings like DADGAD, where the capo can be used to play in keys like G, D, or A without changing the tuning.

Data from various music theory studies indicate that the use of a capo is more prevalent in certain tunings. A study by the University of Western Ontario found that capos are used in approximately 30% of all guitar music, with open tunings and alternate tunings being the most common scenarios. This is due to the fact that these tunings often result in a lower pitch, which can be difficult to play in higher keys without a capo.

Moreover, a capo can also be used to change the tonal quality of a song. For example, using a capo on the first fret in standard tuning can transform the sound to resemble a DADGAD tuning, which is popular in folk and blues music. This versatility makes capos an essential tool for guitarists who wish to explore different musical styles and keys without the need for extensive retuning.

In conclusion, guitar tunings that require a capo are those that are designed to produce a unique sound or to facilitate easier playing in a particular key. Open tunings, drop tunings, and alternate tunings like DADGAD are among the most common scenarios where a capo is used. The use of a capo not only allows musicians to play in various keys but also to change the tonal quality of a song, making it a versatile tool for guitarists.