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Armatures
Almost every two-dimensional artwork has an armature — a division of the format1 that underpins the composition. The vast majority of modern armatures are informal. Example: Sargent's portrait of Lily Millet, below. Like most portraits, this painting's armature is basically a triangle. In addition, Lily is leaning forward with her arms crossed, which causes the upper portion of her body to arc. When an artist arranges people, objects, or shapes on a canvas, an armature is one of the byproducts.
John Singer Sargent, Lily Millet, 1885, oil on canvas, 87 x 67 cm, Metropolitan Museum of Art.
In sculpture, an armature is the rigid metal framework or skeleton that holds the clay or plaster. In two-dimensional art, it's the geometric framework that organizes the composition's spatial elements.
Jan van Eyck's Crucifixion and Last Judgment diptych made aware of the existence of armatures. The underlying geometry is hard to miss — especially in the right panel.
Jan van Eyck, The Crucifixion and The Last Judgment, c. 1440-41, oil on canvas transferred from wood, 57 x 20 cm each, Metropolitan Museum of Art.
In The Last Judgment panel, the skeleton's arcing limbs seem to be defining circles. The area between the skeleton and Christ's feet resembles a square with a circle inscribed in it. The equilateral triangle is suggested by the wings of the angel who is defeating death and the organization of the heavenly congregation above her. The division at the top is the right size for a vesica piscis, in which the center of one circle is at the edge of the other. In traditional Christian art, Jesus and Mary are often depicted inside a mandorla (the cyan geometric figure created by the vesica piscis). In The Crucifixion panel, the throng of people is organized into two groups. Overlapping squares (rabatment) might account for the placement of the thieves' crosses. Perhaps not surprisingly, Christ's cross fits neatly into a golden rectangle.
The spatial divisions of armatures aren't always cleanly executed or verifiable. That is not the case with Ingres' Napoleon I on His Imperial Throne — he left no doubt as to this composition's geometric underpinnings.
Jean-Auguste-Dominique Ingres, Napoleon I on His Imperial Throne, 1806, oil on canvas, 259 x 162 cm, Musee de l'Armee, Hotel des Invalides, Paris.
The inner circles are indicated by the back of the throne. When a circle the width of the canvas is drawn using the same compass point, it divides the format into a square and a golden rectangle. In other words, the format itself is a golden rectangle. This is an armature fit for a deity.

1 The format is the shape or proportions of the canvas or panel.