Kjartan "Kjarri" Sveinsson (] ; born 2 January 1978) is former keyboardist for the Icelandic post-rock band Sigur Rós.
 He joined the band in 1998.
 A multi-instrumentalist, he has also played such instruments as the flute, tin whistle, oboe, guitar and even the banjo, as well as many of the unorthodox instruments that contribute to Sigur Rós's distinctive sound.
Jónsi & Alex is an artistic collaboration between Jón Þór Birgisson (singer for the Icelandic post-rock band Sigur Rós) and his partner Alex Somers.
 They create ambient music as well as visual art.
 The music tends to be more ethereal and experimental than that of Sigur Rós.
"Varúð" (Icelandic for "Caution") is a song written and recorded by Icelandic post-rock band Sigur Rós for their sixth studio album, "Valtari".
 It appears as the third track on the album.
 The song was released as the second official single from the album on August 21, 2012 on a 10" gramophone record in support of their then-current Sigur Rós World Tour.
 It is also notably the first release by Sigur Rós under XL Recordings after the liquidation of EMI and the selling of its subsidiaries, including Sigur Rós' then-current label Parlophone.
Kronos Quartet Plays Sigur Rós is a studio album by the Kronos Quartet, containing two "audience favorites," "Flugufrelsarinn" (by Sigur Rós) and "The Star-Spangled Banner" (trad., arr.
 S. Prutsman after Jimi Hendrix).
 The album is available only as a digital download.
In a Safe Place is an album by The Album Leaf, released in 2004.
 Recorded in Sigur Rós' Sundlaugin studios, it features collaborations in the recording from Sigur Rós' members and quartet Amiina.
The discography of Sigur Rós, an Icelandic post-rock group, consists of seven studio albums, one remix album, three extended plays, one soundtrack album, fourteen singles, nine music videos and two video albums.
 Sigur Rós was formed in 1994 in Reykjavík, Iceland, by singer and guitarist Jón Þór Birgisson, bassist Georg Hólm and drummer Ágúst Ævar Gunnarsson.
Valtari (] , "Roller") is the sixth studio album by Icelandic post-rock band Sigur Rós, released on 23 May 2012 by Parlophone.
 The album was commercially successful going at number eight in the UK Albums Chart, seven in the "Billboard" 200 and number one in Iceland.
 The album was met with positive reviews as well with Metacritic, which assigns a normalised rating out of 100 to reviews from mainstream critics, the album received an average score of 74, based on 36 reviews, indicating "generally favorable reviews".
 Two singles were released from the album, the first being for "Ekki múkk" released on 10" vinyl for Record Store Day, and the second being for "Varúð", as a limited edition 10" to coincide with their 2012 summer tour.
 "Valtari"' s album cover is by Jónsi's sisters, Lilja and Inga Birgisdóttir.
 Inga was also the baby on the cover art for Sigur Rós' debut album "Von".
 It is the last album to feature keyboardist Kjartan Sveinsson.
Sigur Rós (] ) is an Icelandic post-rock band from Reykjavík, who have been active since 1994.
 Known for their ethereal sound, frontman Jónsi's falsetto vocals, and the use of bowed guitar, the band's music is also noticeable for its incorporation of classical and minimalist aesthetic elements.
 The band is named after Jónsi's sister Sigurrós Elín.
Jón Þór "Jónsi" Birgisson (] ) (born 23 April 1975) is the guitarist and vocalist for the Icelandic post-rock band Sigur Rós.
 He is known for his use of a cello bow on guitar and his falsetto or countertenor voice.
 He is blind in his right eye from birth as a result of a broken optic nerve from the brain.
 He is openly gay.
 Apart from Sigur Rós, Jónsi also performs together with his partner Alex Somers as an art collaboration called Jónsi & Alex.
 They released their self-titled first book in November 2006, which was an embossed hardcover limited to 1000 copies, along with their first album, "Riceboy Sleeps", in July 2009.
The Owl Service was an English alternative folk music collective formed in 2006 by multi-instrumentalist Steven Collins (who led the band for its entire 10 year duration), named after the 1967 novel by Alan Garner.

Question: What activity do both The Owl Service and Sigur Rós have in common?
Answer: music