[2] Around July 1967, the Gibb brothers sang backup vocals on Johnny Young's cover version of "Craise Finton Kirk Royal Academy of Arts" from Bee Gees' 1st, released as a single the following month.

[3] Also in July 1967, the Gibb brothers sang backup vocals on Oscar's cover version of "Holiday", with an orchestral arrangement by Bill Shepherd.

[3] Around July or August, Barry and Robin wrote "Cowman, Milk Your Cow", which was recorded by Adam Faith, with the Gibb brothers on background vocals, Russ Ballard and Pete Salt on guitars (though one source would suggest the latter was in fact Fleetwood Mac's Peter Green), Milt Rogan on bass and Bob Henrit on drums.

Venturing to Denmark Street (known as London's Tin Pan Alley), the Bee Gees booked Central Sound for 17 July, quickly cutting several tracks.

"Words", Sinking Ships", "Barker of the UFO" and "Sir Geoffrey Saved the World" were recorded during this album's sessions.

The leftovers from the Horizontal sessions, "Out of Line", "Ring My Bell", "Mrs. Gillespie's Refrigerator", "Deeply Deeply Me", "All My Christmases Came at Once", "Thank You for Christmas" and the medley "Silent Night/Mary's Boy Child" (the latter erroneously listed as "Hark the Herald Angels Sing", a different Christmas carol from which the phrase had been lifted for the lyric) were released on the 2006 remastered version of Horizontal on Reprise Records.

On the group's return to England, they recorded their third BBC session at the Playhouse Theatre at London's Northumberland Avenue with a 19-member orchestra under the direction of Bill Shepherd.

After Horizontal was released, it seemed to get mixed reactions from fans, probably owing to it having a darker lyrical tone and a heavier musical sound than Bee Gees 1st.

Horizontal was considered the heaviest album ever recorded by the Bee Gees, due for the most part to an increased influence asserted by guitarist Vince Melouney and drummer Colin Petersen.

The album cover for the American release was changed slightly in that the photo of the group was reversed and was adorned with an oval picture frame.

[2]Allmusic's Bruce Eder describes some songs in the album: "World" as 'a poignant, even somber yet gorgeous ballad filled with clever lyrics, and highlighted by a quavering Mellotron accompaniment, a very close grand piano sound and twangy fuzz-tone guitar' and "And the Sun Will Shine" as 'an even more serious, regretful ballad that is bearable because it is also prettier than "World"', and "Harry Braff" as 'cheerful'.