A discussion group on the calculation of petrologic phase equilibria is at groups.io/g/PerpleX. The list was started on February 12, 2009, by Riia Chmielowski (LTU); Vincent van Hinsberg (McGill) became the list owner as of November 28, 2019. With regard to Perple_X, the group provides a forum to compensate for Perple_X's abysmal documentation. More importantly, the group serves as a venue for the exchange of information on subjects where I have little expertise, e.g.: on the choice of solution models and thermodynamic data bases; on the optimal choice of run-time options; and on ways to use the programs efficiently. JADC, 11/19.