Símbolo que se asemeja a la letra Griega "tau" (T) 

1- Es The standard raised by Moses to elevate the brazen serpent, 

2- and the mark made by the Israelites in blood on their doorpost is thought to have been the tau. 

the tau cross is also the emblem of St. Anthony of Egypt.

See also

 http://www.shrinesf.org/francis10.htm