Early Trinity Symbols in Chrismons

A Trine Compass, described by Chaucer.

       Triangle is said to be the oldest symbol to represent the Trinity. The importance of the Triangle as a Christian symbol is perhaps not fully realized, let the value of its significance be impressed by quoting from an authority in art. "Triangle. An equilateral triangle is a symbol of the Holy Trinity, and many figures in Christian ornament are constructed on this principle, as types of that mystery. The equilateral triangle is found in the most beautiful arches, in the proportions of the churches themselves, and next to the cross, is the most important form in Christian design." Fairholt's Dictionary of terms in Art
       The Circle is so well known as a symbol of eternity, or expressing that which has neither beginning nor ending, that its presence with the triangle seems to need no explanation.
       One form "apt and venerable" the trine compass (as it was called by Chaucer), or a circle inscribed within an equilateral triangle: denoting the co-equality and co-eternity of the three divine persons in the ever blessed and undivided Trinity" During the first eight centuries of Christian art, the use of the Triangle was not well developed. In that period the three persons of the Trinity were represented in art, but separately. The first time they seem to have been placed together, was in the fourth century, and that representation consisted of "the Hand, the Lamb, and the Dove, " which is said no longer to exist. There is instanced a drawing of a triangle on one of the grave-stones in the Catacombs, in which the monogram of the name of Christ is placed. Elizabeth Clifford Neff

• Scriptural References for The Holy Trinity here