Vitruvius wrote in the decade from 35 to 25 BC his masterpiece "De architectura", conceived as a summary of the architectural knowledge of his time and previous ones. Divided into ten books, the second one was chosen by the author to expose extensively the building materials and construction techniques used in his time. Enunciated all of them with the generic word "Opus", the second word serves to define each particular technique. Let's meet them...
Opus Quadratum. It is called by this name to the construction technique with ashlars --photos 1,1bis and 2--, well known before Rome, although it is under the Latin aegis when it reaches its zenith in Western Europe. This is evidenced, certainly, by the many examples of structures built in Roman times employing this technique: both civilian and military ones, residential, palatial or religious.
Photo 1.- Alconetar bridge. Garrovillas de Alconetar, Cáceres, (Spain). 2nd century AD. Opus Quadratum placed alternating headers and stretchers. Work of average quality, typically provincial.
Photo 1Bis.- Splendid example of high quality Opus Quadratum in the Roman theater of Miletus (Turkey).
Greek origin technique, the Hellenes raised the entire structure, what includes not only the outer walls but also the core, with ashlar. However the Roman architecture, more practical than the Greek, replaced from the beginning (end of the fourth century BC) the ashlar core by another one, much cheaper, of coarse masonry aglomerated with lime mortar in what is the first example of use of lime mortar as binder (until then the lime mortar had been used exclusively as a coating). This technique, known according to the classical sources by the name of Emplecton, spread quickly from Italy throughout the Roman Empire, having persisted in use until the final decades of the nineteenth century when the development of Portland cement and modern concrete withdrew it definitely.
Photo 2.- Ancient city of Cáparra. Villar de Plasencia, Cáceres (Spain). Splendid water well, zenith of the Opus Quadratum --curved ashlars--. 2nd century AD