Rabatment is a method of composition in which squares are overlapped in a rectangular format. Squares with sides equal to the short side of the rectangle are placed over each of its long sides. Where the squares intersect or overlap, important compositional elements are placed. In her book, Geometry of Design: Studies in Proportion and Composition, Kimberly Elam uses a painting by Degas to show how rabatment works.2
Edgar Degas, Racehorses at Longchamp, 1871, possibly reworked in 1874, oil on canvas, 34 x 42 cm, Museum of Fine Arts, Boston.
The crossing diagonals indicate the red squares of the primary rabatment. The black squares in the area of overlap are a secondary rabatment. Some artists believe that dividing the picture plane this way facilitates the creation of a harmonious design.
Rabatment was used to design sacred art in southern Europe from the late medieval period through at least the early Renaissance. It doesn't seem to have been used in northern Europe — at least not until Jan van Eyck superimposed it on the The Adoration of the Lamb.
Jan van Eyck, The Ghent Altarpiece: The Adoration of the Lamb, 1432, oil on panel, 138 × 242 cm, St. Bavo's Cathedral, Ghent.
As the armature shows, the baptismal fountain fits perfectly into the rabatment's area of overlap. Within this overlap is a golden rectangle that begins at the format's midline and ends at the bottom of the fountain. Was it intended? The altar on which the lamb stands is also a golden rectangle. (I've included circles and ellipses in the armature to call attention to Van Eyck's predilection for using simple geometric shapes to organize his compositions.)
The bottom image is an infrared reflectograph of The Adoration of the Lamb.3 The lamb's altar was included in the underpainting, but the baptismal fountain is dark and hard to make out.
According to J. R. J. van Asperen de Boer, a Dutch physicist who is an expert on Northern Renaissance underpaintings, the fountain was not part of The Adoration of the Lamb underpainting.4 A stereomicroscope has shown that there is green paint under it. Also, it does not appear in X-radiographs. Did van Eyck add the fountain after traveling to Spain and Portugal? Paul Coremans, a Belgian scientist cited by van Asperen de Boer, believed that the Mediterranean scenery indicates that he completed the painting after one of those trips.5
Robert Campin, the Master of Flemalle, was twenty years older than van Eyck and an established artist by his early thirties. Rabatment does not appear to have been one of the techniques that he used in designing his paintings. His armatures typically consist of a few simple geometric shapes. The Entombment, below, predates the Ghent Altarpiece by at least seven years.
Robert Campin, The Seilern Triptych, also known as Entombment, before 1425, oil on panel, 60 × 49 cm (central panel), Courtauld Institute, London.
The mandorla was probably no accident. The Merode Altarpiece, as well as Rogier van der Weyden's Descent from the Cross, also seem to have been organized around it. (Van der Weyden is thought to have been Campin's student.)

2 Kimberly Elam, Geometry of Design: Studies in Proportion and Composition, New York: Princeton Architectural Press, 2001, 50-51. Kimberly Elam is chairperson of the Graphic and Interactive Communication Department at the Ringling School of Art and Design in Sarasota, Florida.
3 The infrared reflectograph is from the website Closer to Van Eyck: Rediscovering the Ghent Altarpiece.
4 J. R. J. van Asperen de Boer, "A Scientific Re-examination of the Ghent Altarpiece." Oud Holland 93, no. 3 (1979): 194.
5 Ibid., 141.