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Among alternative tunings for guitar, a major-thirds tuning is a regular tuning in which each interval between successive open strings is a major third. Other names for major-thirds tuning include major-third tuning, M3 tuning, all-thirds tuning, and augmented tuning. By definition, a major-third interval separates two notes that differ by exactly four semitones.
The Spanish guitar's tuning mixes four perfect fourths and one major-third, the latter occurring between the G and B strings:
This tuning, which is used for acoustic and electric guitars, is called "standard" in English, a convention that is followed in this article. While standard tuning is irregular, mixing four fourths and one major third, M3 tunings are regular: Only major-third intervals occur between the successive strings of the M3 tunings, for example, the open augmented C tuning.
For each M3 tuning, the open strings form an augmented triad in two octaves.