The shafts of the Pantheon's portico columns are composed of granite monoliths, forty Roman feet tall, which were excavated from two Egyptian quarries. The eight light gray columns of the front row of the portico originated from the imperial quarry at Mons Claudianus, while the pink column shafts of the middle and back rows (four in each row) probably came from the Assuan region. The sixteen colossal shafts, each weighing about fifty metric tons, were brought by ship from Egypt to Rome. Although they were by no means the largest monoliths used in Roman architecture (indeed, there are indications that the portico of the Pantheon was originally intended to be furnished with even larger columns); their transportation and erection would nevertheless have posed a serious challenge to the architects of the time.

Although the architectural and historical significance of the columns of the Pantheon's portico are undisputed, the question of how they were designed remains a moot point. Particular attention has been given to the design problem of the so-called “entasis,” the slight vertical curvature of the column shaft. Gorham P. Stevens was the first to explore systematically this architectural feature in Roman architecture. To determine the design principles of Roman columns, he measured and geometrically analyzed columns from a variety of well-known Roman buildings, coming to the conclusion that Roman column design could reach a considerable degree of complexity. For example, Stevens believed that the Pantheon's portico columns exhibit a profile that is based on two tangent hyperbolas. More recently, the design and execution of the entasis has been reassessed by Mark Wilson Jones, who seriously doubts Stevens’ results. Nonetheless, his counterproposals remain speculative in many respects, principally because in many instances he was not granted access to re-measure the examples analyzed by Stevens.

The current study, which is based on the data gathered for the Bern Digital Pantheon Project, analyzes the geometry of the Pantheon's portico columns with the aim of developing a hypothesis on the geometrical design of the entasis. The model developed to explain the measured column profiles is based on the parameters that Vitruvius gives for the design of columns in his treatise, De architectura libri decem (Ten Books on Architecture), and on the scale ratios of the construction drawings discovered at the temple of Apollo in Didyma, Turkey.