The Greek cross is a very early form of the Christian cross. Often found in conjunction with the ankh, the Greek cross predates the Latin cross and was not intended to represent the cross of the crucifixion. Instead, it stood for the four directions of the earth, representing the spread of the gospel, and the four platonic elements. The Greek cross was a popular floor plan for eastern churches at one time. This style of cross dates to ancient Babylon, where it was a symbol of the sun-god Shamash. It was also popular in pre and post Christian Celtic art.
Alexandrian Cross between ankhs
The Greek cross also has a hidden, esoteric meaning. The four directions correspond to the the fixed cross of the zodiac (Leo, Taurus, Scorpio, Aquarius), as represented by the four evangelists; the cross can be divided into thirteen-segmented cube, representing the twelve apostles- and of course, the twelve zodiac signs, as well as spirit or quintessence.
This cross is often referred to as the cross of earth, and is a basic element of many alchemical symbols.
Some variations on the Greek cross:
A Greek cross within a square is usually taken to represent earthly affairs or the temporal authority of the Church.
This style is called “voided,” for the empty space within, and again symbolizes the four directions; it is sometimes interpreted to be composed of four turned greek gammas (equivalent to the letter “g:”). This is also called a crux gammata or cross gammadion, a name it shares with the swastika when used in heraldry.
This cross is used in the Eastern and Greek Orthodox Church; the letters make up the Greek, Phos Zoe, “light and life.”
Assyrian cross, 3000 BCE