Legend has it that a gigantic demon, Vastu Purusha, arose from a drop of Shiva's sweat and ravaged the world. In their desperation, the gods got together and collectively pinned him down thus saving the world and bringing about order where there was disorder. Each of the gods was assigned a specific seat or pada, with Brahma occupying the centre and 32 deities placed in the peripheral squares according to their importance. The Mandala is a cosmic diagram of the subjugated Purusha and shows him with his head towards the north-east and his feet towards the south-west.

The legend of the demon Vastu Purusha and his eventual subjugation by various gods was a beautifully laid out metaphor to illustrate how one should plan a house. The Mandala, like the one at left, usually comprised 81 squares, though the number could as well have been 64.

Not surprisingly, the positions of the different gods in the Mandala are the basis for related activities in the house. For example, sinceAgni, the god of fire inhabits the south-east corner, that is considered the ideal place to have a kitchen. Brahma, the supreme being who inhabits the large central square, is also representative of the dimension above ground level. This portion is therefore left open to sky as a courtyard - a very practical way of ensuring adequate light and ventilation within the dwelling. The courtyard in general, is almost always cool and shaded, so becoming the focus of all household activity.

The number of squares in the Mandala follow a geometric progression of 1, 4, 9, 16, 25, 36, 49, 64, 81,... This goes on till 1024 (which is, incidentally, the square of 32 and used for town-planning). The usual numbers for residential structures are 64, or the more popular, 81.

The Mandala provided a useful grid for the Sthapati to assign specific locations to different rooms and also helped him in determining their relative proportions. The measurements were made in anthropomorphic units like angula (finger) or hasta (hand); a system that is slightly out of place when the materials we buy today are quantified in inches or centimetres.