The Tau cross, named after the Greek letter it resembles, is a very old symbol and is often used as a variant of the Latin, or Christian cross.
   The Tau Cross in Christianity dates back since the latter's beginnings. Today, in the Catholic Church it is used in reference to Saint Francis, who proclaimed to his fellow friars in his hometown of Assisi (Italy) that their monastic habit was the Tau Cross. When the arms of the one wearing it are outstretched to the sides, it roughly creates an image of the Tau Cross. If one then includes the body of the one wearing it in the imagery, the wearer becomes a living, walking crucifix. St. Francis is said to have reminded his followers of their role by reminding them of this imagery.
   There are many other Orthodox churches that use the TAU in their liturgy services as well. The TAU cross is often used as a decorative symbol on liturgical vestments, stoles, banners etc.
The lovely painting pictured by Rogier van der Weiden is of Joseph of Arimathaea Supporting the Dead Christ. In the background we see a TAU cross used as a crucifix.