The square cross is sometimes called the ‘primordial cross‘ because it appears in diverse cultures and has deep symbolism. Symbologists say the square cross denotes a spatial orientation—a center with forces entering in and moving outward.
The square cross also links the circle and the square. It has been a symbol for the meeting of heaven and earth (China) and for an alleged ‘umbilical chord’ of the cosmos.¹
The square cross has been connected to the biblical idea of Paradise due to the four rivers flowing outward from the Garden of Eden. Moreover, it symbolizes a meeting point between the living and the dead.
But this just touches on countless motifs associated with this cross.
In Christian usage, we find the square Greek Cross and a modified form in the Jerusalem Cross, an emblem of the early Christian Crusaders. The insignia of Godfrey de Bouillon, the first ruler of Jerusalem after defeating the Muslims in 1099, was the Jerusalem Cross, which he wore at all times.
¹ For examples, see http://www.mesoweb.com/features/fabric/textindex.html