Open form is a term coined by Heinrich Wölfflin in 1915 to describe a characteristic of Baroque art opposed to the “closed form” of the Renaissance (Wölfflin 1915, chapter 3). Wölfflin tentatively offered several alternative pairs of terms, in particular “a-tectonic” and “tectonic” (also free/strict and irregular/regular), but settled on open/closed because, despite their undesirable ambiguity, they make a better distinction between the two styles precisely because of their generality. In an open form, which is characteristic of 17th-century painting, the style “everywhere points out beyond itself and purposely looks limitless”, in contrast to the self-contained entity of a closed form, in which everything is “pointing everywhere back to itself” (Wölfflin 1950, 124). In general, the closed compositions of the 16th century are dominated by the vertical and horizontal, and by the opposition of these two dimensions. Seventeenth-century painters, by contrast, de-emphasize these oppositions so that, even when they are present, they lose their tectonic force. The diagonal, on the other hand, becomes the main device used to negate or obscure the rectangularity of the picture space (Wölfflin 1950, 125–26).
