Produced by Lenny Waronker and released in July 1967, it was the band's first album to include songs that vocalist Sal Valentino and guitarist Ron Elliott composed together.

The band incorporated fantasy elements and surreal characters into the album's song titles and lyrics, and worked with a variety of session musicians to create Triangle's psychedelic musical style.

Triangle reached number 197 on the Billboard 200 albums chart and received mostly positive reviews; critics commended Elliott as a songwriter and compared Valentino's vocals to those of Bob Dylan.

[6] In contrast with the band's generally straightforward recordings for the Autumn label, Triangle's lyrics are more abstract, containing Tolkienesque fantasy elements[10] and dream-like characters, such as the gypsy in "Only Dreaming Now", the "Painter of Women", "The Keeper of Time", and "The Wolf of Velvet Fortune", as well as the destination of "Magic Hollow".

[11] Having regained artistic freedom in the studio, and with the band no longer touring, Elliott and Valentino focused on creating an album of songs which were written and recorded specifically for that purpose.

[7] Session musicians included guitarist James Burton, drummer Jim Gordon, bassist Carol Kaye, and Van Dyke Parks, who played harpsichord on "Magic Hollow", adding to the album's psychedelic musical style.

[19] Music journalist Jon Savage named the song "Magic Hollow" in his list of the "100 Greatest Psychedelic Classics" in the June 1997 issue of Mojo magazine.

[20] In 2006, Joel Selvin of the San Francisco Chronicle called the album a "cult classic",[21] while SF Weekly's Justin F. Farrar remarked that it "has aged far more gracefully than that almighty concept record from '67, Sgt.

"[22] Brendan McGrath of The Rising Storm wrote in a 2007 review that "Triangle has everything: it's a tightly produced country record that is rooted in rock; it's straight and folky and underlined by psychedelic imagery".

[23] Author Tom Moon selected the album for his 2008 book, 1,000 Recordings to Hear Before You Die, in which he wrote that Triangle "captured the crisscrossing events of 1967" and was "the rare bridge between the sunny straightforwardness of mid-60s pop and the fuzzy opaqueness of psychedelia".