The Eastern Orthodox Cross (also known as the Byzantine, Greek Orthodox or Russian Cross) is a variation of the Christian cross, commonly found in some Slavic Eastern Orthodox Churches, as well as in the Eastern Catholic Churches of Byzantine rites, and in the Society for Eastern Rite Anglicanism.
It has three cross beams and is distinctly different from other Christian crosses.
It is believed that the top beam, also found on the Patriarchal cross, represents the plaque with the INRI inscription, though such upper beam rarely has any inscription (it is just symbolic of a titulus).
The bottom beam represents a footrest, a foot-support (suppedaneum) that seems to have appeared first in Eastern Christian art in the 6th Century. While the purpose of the suppedaneum was to support the weight of the body, it unclear if such a device was part of Jesus’ cross.
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Pastoureau, 13-67. Neubecker, 106-107.
Murray, 296-297
Wikipedia
www.crosscrucifix.com