**1. Introduction**

The study of architectural history is and always has been one of the fundamentals in each architect's education; up to the 1920's circa also the design classes were embedded by the replica of the Greek temple orders and the imitations taken from ancient reference books such as Vitruvius, Palladio, Viollet-le-duc and many other. This changed after the 1920's in favor of a more multidisciplinary and technical orientated training, promoted by "Ornament and Crime" (A. Loos, 1908), "Form follows function" (R. Sullivan, 1900 ca.) and other design maxims. It caused a proportionate impoverishment of building history knowledge and of traditional design methods. One said the architect-artist's mind had to be liberated from all historic ballast, and

should be able to create within his free individual creativity. This is a discussable principle with potentially quite negative consequences as architecture is not only a question of artistic creativity or aesthetical harmony, nor a pure functional or technical discipline: "Architectura … nascitur et fabrica et ratiocinatione …" ("Architecture is born by craftsmanship and balanced rationality", Vitruvius, I°sec. b.C.) in ([1], I,1). Architecture (with capital) needs both approaches and apart from the Vitruvian "utilitas, venustas et firmitas [1]", Architecture always had an existential and universal dimension dealing with bringing sense and structure in the surrounding space, including physical communication with meanings and messages' to the observer [2].

The often poor knowledge on historic design criteria nowadays, inevitably leads in many cases to a considerable loss of 'sense' and a different type of 'meaning and message' in contemporary projects. Many heritage buildings get their conservation status because of tangible cultural and historic characteristics, and in many cases it is completed with a large intangible content expressed through symbolisms and allegories. Unfortunately, very often this symbolism and allegories get lost today as man is not familiar any more with the ancient allegorical languages. Also the other way around, modern design rarely uses those so called 'old-fashioned' allegorical indications in such speechless but most effective communication between designer and observer. Medieval buildings are particularly representative for the presence of this mostly forgotten intangible communication content, expressed through the symbolism of form, number, proportion, material or color. Based on the analysis of some representative medieval buildings, this chapter illustrates and tries to detect such design indicators to inspire the contemporary designer, not suggesting a flat imitation but a personal modern interpretation and use of the very same ancient design indicators. The two mayor instruments to all kind of allegorical allusions in medieval design are the geometry of the architectural form and the arithmetic's within the different quantities and dimensions.

This book aims contributing in sustainable construction. The easy re-use or reconversion without great structural change or loss of architectural identity is part of all sustainability and certainly one of the most crucial assignments today. Recent experiences on the reconversion of existing fabric or the recuperation of ancient abandoned structures, mostly for evident economic reasons, have proved abundantly that reconversion or recuperation is much easier and less invasive with ancient well-modulated traditional buildings as it is the case with some contemporary building or probably shall be with one of the super eye-catching designed ones, created by great archistars as e.g. the Bilbao Guggenheim Museum or the Baku Heydar Aliyer Center. Certainly, those superlatives are strong signs of digital design and technical knowledge, but their quality remains onesided, limited to never seen forms and materials. They do not show great flexibility nor long lasting esthetic pleasure; any probable later intervention, as proof of sustainable (re-)use, risks to damage considerably their actual identity. Society needs avant-garde, but this has to be applied with cure and caution. Contemporary design should reconsider the historic canons, take profit of the three thousand years' experience, evaluate and integrate the old principles for harmony and sustainable use in the modern design algorithms to guarantee qualitative architecture and long lasting construction.
