The prime member, the Cross, had been introduced to Christian art in the 4th century as the "crux invicta", a symbol of victory. As a group they have a long tradition in iconography, dating back to the 9th century; the Utrecht Psalter of 830 is an example, though the only one from the Early Middle Ages known to Gertrud Schiller. This reflected an increase in theological interest in the sufferings of Christ at the time. The Middle English poem "Arma Christi", which appeared before the end of the 14th century, exists in fifteen manuscripts, attesting to its popularity, of which seven are engrossed in highly unusual scroll form, designed to be displayed in church as a pictorial aid to public devotion; manuscripts of "Arma Christi" are generally accompanied by illustrations of the instruments, viewing of which, according to the texts, granted indulgence of a certain number of days in Purgatory to come.