This invention describes a primary construction item. This item is formed from two longitudinal support sections (10, 11) of identical length and height, and possibly of different width, separated from each other and interconnected by a longitudinal joining section (1C) of the same length, linking the two support sections (10, 11) to form in cross-section a bridge with support sections (10, 11) forming the pillars, and an arch (1D) being formed between these two support sections (10, 11) and the joining section (1C), the geometric configuration of external face (1B) of joining section (1C) being similar to that of the configuration of arch (1D) such that external face (1B) of joining section (1C) of another construction item may fit exactly in said arch (1D), the external face (1B) of the joining section having a groove (9) and arch (1D) having a rib (9') designed to ensure precise dosing of the bonding material used.
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