The triangle of art is theprotected space outside the magic circle, into which spirits are compelled to appear in Solomonic ritual magic. Typically, the central circle is inscribed with the sigil (seal) of the spirit to be invoked. The usual form is of a triangle, circumscribed with various words of power, containing an inner, blackened circle.
The purpose of the triangle is to contain the manifested entity. In some cases, the triangle is created as a physical object; sometimes, the central circle is replaced with a black scrying mirror.
The word sigil means a seal or device (like a personal seal, a printer's mark, etc.), but the word today usually denotes a magical sigil, a glyph used in ritual or sympathetic magick as a focus, or for summoning angels, demons, or spirits. Sigils as the signature of 'mark' of a spirit began in antiquity and reached a peak of popularity in during the Renaissance. The seal or signature of an angel, spirit, or demon was thought to contain the essence of that being's name, and give one power to summon that entity.
There are several methods of devising sigils for this use, usually involving special arrangements (planetary squares, Enochian tables, etc.) of letters or numbers, and tracing the desired word over this pattern to reveal a symbol (the detailed center of the Golden Dawn Cross is such a pattern), or reducing alphabetical characters into a single glyph.
Scrying is a form of divination which involves prolonged gazing into a crystal, a pool of water, a black mirror, or similar surface, in order to obtain visions. The famous prognistor Nostradamus was said to have favored this method, using ink as a medium.