III & IV: Air & Earth is the sixth studio album by American rock band Thrice.

It consists of the final two volumes of The Alchemy Index, a four-disc concept album that was split between two releases, the first in October 2007 and the second in April 2008.

[2] The disc's overall atmosphere recalled the Appleseed Cast and The Devil and God Are Raging Inside Me (2006)-era Brand New,[5] utilizing reverb and delay frequently.

[8] Eddie Breckenridge cited artists he was listening to at the time as inspiration for the disc, namely Sufjan Stevens, Sigur Rós, and the Mercury Program.

[9] "Broken Lungs" is a mid-tempo track with delayed guitar parts,[10] alongside bell chimes and a glockenspiel.

[11] "A Song for Milly Michaelson" is a minimalistic track with a repetitive guitar part and gentle vocals from Kensrue,[11] which recalled Pedro the Lion frontman David Bazan.

[11] "Daedalus" discusses the Greek figure of the same name and his son Icarus, which the band previously talked about with "The Melting Point of Wax" from their third studio album The Artist in the Ambulance (2003).

[18] "Moving Mountains" sees Kensrue rehash 1 Corinthians 13, which talks negatively of any act that lacks love.

[11] The song almost ended up on Please Come Home; it was based on a guitar riff that Eddie Breckenridge came up with, which the band re-wrote and expanded on.

[2] Donia Lee contributes additional vocals to the track,[20] which also features woodwind instrumentation and finger snapping.

[11] Thrice covered the Frodus song "The Earth Isn't Humming", which they turned into a slow-tempo country-esque track, complete with banjos.

[5] It features backing vocals from Aushua, Nick Bogardus, Brent Kredel and Brett Williams,[20] alongside trumpets and a repetitive piano line.

[36] A Legion of Doom remix of "Broken Lung" appeared on the Underworld: Rise of the Lycans soundtrack in January 2009.

[38] AllMusic reviewer Anthony Tognazzini wore that the "tight, technically complex instrumental prowess of the band is still on display, but the mood is much more atmospheric" than the Fire and Water discs.

[40] The staff at Ultimate Guitar said "once again the band have absolutely nailed the sonic theme for each element", though added that these discs paled in comparison to Fire and Water, something that Tim Karan of Alternative Press agreed with.

[11] Melodic staff writer Tom Spinelli said Air displays the band's skill at composing "these emotive, passionate, heartfelt songs, breaking them away from their post hardcore past".

Earth "starts us off with a more laid back feel whilst maintaining that melodic energy of The Alchemy Index [project] as a whole".