The sound of a melody harmonized in thirds (intervals of a third) will be familiar to most people. It is simply two notes at the interval of a third apart, played simultaneously. As a compositional tool, thirds are an invaluable way of enhancing a melody. Here is the D Major scale played in thirds.
The following melody is in the key of D Major and demonstrates a melody played in thirds over an alternating bass using open D notes.
In the early part of the 20th century, guitar manufacturer C.F. Martin released an acoustic guitar with a larger, deeper body than most existing guitars.
Around the same time, the British Navy launched a battleship that was so big it would fear nothing. It was called "HMS Dreadnought". Martin thought this would be a good name for his new guitar.
The guitar sounded great and the name caught on. Today, the dreadnought is the most commonly used type of acoustic guitar in the world.