Jeroen Paul Thesseling (1971) followed his first classical violin lessons at the age of seven and started his study bass guitar in 1988 at the ArtEZ School of Music in Enschede, the Netherlands. After playing metal and jazz in his late teenage years he collaborated with the Dutch group PESTILENCE from 1992 till 1994, recording their jazz-fusion influenced album 'SPHERES' (1993). Since 1995 he experimented with microtonal octave divisions, which resulted in 'HAFNIUM' - study in 72-tone equal temperament (1999) and 'ARGON' - study in 18-tone equal temperament (2000). In 2005 Jeroen recorded his first fretless work with the studio project ENSEMBLE SALAZHAR. Since that same year he plays exclusively fretless bass. From 2007 till 2011 he collaborated with the German group OBSCURA with whom he recorded the critically acclaimed studio albums 'COSMOGENESIS' (2009) and 'OMNIVIUM' (2011). During the period 2009-2012 he rejoined PESTILENCE after a 15-year break, recording the album 'DOCTRINE' (2011). Since 2011 he plays 7-string fretless basses built by German bass luthier Warwick, which are strung with a subcontra F# tuning. Jeroen has been touring the world. In 2014 he founded the jazz-fusion project SALAZH TRIO with whom he released their debut album 'CIRCULATIONS' (2017), featuring legendary drummer Horacio 'El Negro' Hernández. Late 2019 he created the fusion-world-metal group QUADVIUM together with fellow fretless bass player Steve Di Giorgio. In April 2020, after a nine-year break, it was revealed that Jeroen rejoined the German group OBSCURA as permanent bass player. With them he recorded their sixth studio album 'A Valediction' (2021).