This study focusses, in façade and in plan, on the horseshoe arch, the hallmark of architecture in the first flowering of the Middle Ages, and on the recessed arch on piers, its more or less unrecognised contemporary. The geographical extent of the occurrence of these two types of arch has been defined by the periodic enlargement of the area where they are really concentrated: Spanish Catalonia and French Languedoc-Roussillon. Applied often to dating a building, and becoming involved as a result in a stylistic epistemology, as well as being the targets of the entrenched dogmas of a vast historiography, these pieces of evidence are here studied on the ground in their architectural context so as to juxtapose fieldwork with the theories on their deployment developed a century ago. Given the spread of developed mediaeval arches, this work endeavours to place their corpus in a wider general study which clearly cannot depend on an exhaustive survey, but which is able to suggest, nevertheless, that the realm of the Moorish arch transcends the territorial and chronological limits of Visigothic, Mozarabic, Carolingian or Islamic culture. The origin of the recessed arch on its piers, for which we propose the term «mushroom shape» displays an undeniable origin in antiquity. Its diffusion in time and space coincides with the horse-shoe throughout the vast Mediterranean ambit. The corpus of the area studied brings together 98 buildings which preserve in their structure the Moorish arch and/or the recessed arch on piers. An analysis of arch assembly, incorporated in the overall study of a building’s construction, confirms that the Moorish design or that of the recessed arch on piers concerned not only arches but was inherent in a general building process observable in the raising of the vault, or of any transverse arches or in openings, doorways or windows. The endurance of these same building procedures for centuries, for minor works such as small country chapels, as for the great abbey church of Saint- Michel de Cuxa, testifies to a conservative art associated with basic techniques.