the double cross of lorraine

And the sigil used by Crowley was also employed by ancient alchemists as a device whose meaning was literally "very poisonous." In fact, the symbol was commonly affixed to containers of toxic substances in Europe as recently as the mid-twentieth century. To the alchemists, of course, the symbol and its very meaning had far more esoteric connotations. To them, poison represented an agent of transformation, a vehicle for the reconciliation of opposites. And there is an alchemical myth about a poison which for most men is extremely deadly, while for the elect it confers mastership and absolute power. Echoes of this idea recur as a motif in various aspects of the Grail lore. It would seem that the alchemists accorded this symbol very much the same meaning attributed it by Crowley, which in turn echoes what the Cross of Lorraine embodied for Rene d'Anjou. It is little known, but the Cross of Lorraine was also the official emblem of the Knights Templar.

Though they are more frequently associated with the symbol of the red equilateral cross, their true symbol first and foremost was the Cross of Lorraine, and many Templars awaiting death at the stake pursuant to the Friday the 13th persecutions drew the emblem on the walls of their cells. There is still a degree in certain rites of Freemasonry called the Knights Templar, whose symbol is the Cross of Lorraine. It is interesting to note that even the less esoteric-looking equilateral cross used by the Templars had essentially the same meaning: that of the union of opposites, the intersection of creative force and destructive force, or the union of male and female principles. It was a fundamental occult symbol, and it was in deference to the idea which it embodied that medieval occult rituals were often held at crossroads.

The Cross of Lorraine is far more explicit in it's iconography: the bar above mirroring the bar below, both of which are symmetrically affixed to a central pillar that provides balance and equilibrium. As above, so below is, after all, a Hermetic maxim. And the three bars of which the cross is composed echo Eliphas Levi's concept of the true trinity. Levi posited that the world is governed by two primordial forces, one creative, the other destructive. The equilibrium between these two forces constitutes a third force, and the union of the three constitutes what some might refer to as... God.

The central role of the principle of equilibrium in various occult arts cannot be overstressed. For Master Mason Albert Pike, equilibrium represented the Royal Secret, the pivotal principle upon which the universe is ordered. And Pike's specific designation of equilibrium as a royal secret is of particular significance because, as previously mentioned, Hermeticism has traditionally been known as the Royal Art, and its central tenet is essentially that of equilibrium.
[extracted from http://www.thevesselofgod.com/crosslorraine.html]

In typography the double cross is called double dagger, double obelisk and diesis.
In botany a balanced cross (equal length bars on equal distances) is used to mark very poisonous plants.
Used to symbolize checkmate.
The "Cross of Lorraine" symbol appears in Unicode as U+2628 (☨) or U+2021 (‡) and in HTML as ‡ (‡).
The Cross of Lorraine design was used by silversmith during the Fur Trade in North America( 1680 -1820) When certain^[who?] Native American group first saw the cross they thought that it was a dragon fly, which in their culture is a sacred being that took prayers to the Great Spirit on their behalf.^{[citation needed]}