A ladder of thirds (coined by van der Merwe 1989, adapted from Curt Sachs) is similar to the circle of fifths, though a ladder of thirds differs in being composed of thirds, major or minor, and may or may not circle back to its starting note and thus may or may not be an interval cycle.
Triadic chords may be considered as part of a ladder of thirds.
It is a modal frame found in Blues and British folk music. Though a pentatonic type, the melodies come "into being through piling up thirds below and/or above a tonic or central note." (Middleton 1990, p.203) [while pentatonic scales are analyzed as piled fifths]
They are "commonplace in post-rock 'n' roll popular music - and also appear in earlier tunes". (Middleton 1990, p.203) Examples include The Beatles' "A Hard Day's Night", Buddy Holly's "Peggy Sue" and The Who's "My Generation", Ben Harney's "You've Been A Good Old Wagon" (1895) and Ben Bernie et al.'s "Sweet Georgia Brown" (1925).