Creating a numerical model of a reinforced concrete building involves a large number of decisions, each carrying assumptions and limitations. Knowing these limitations is essential to define a model that fits the scope of the analysis. This keynote presents a collection of fundamental questions that any numerical model of a pre-code RC building should be able to answer in order to define its own boundaries of validity and earn the confidence of whoever uses it. The questions are organised around four themes: the mathematical formulation of the model (material models, dimensions, plasticity, hysteresis, damping, integration schemes); the representation of the physical building (differentiate between structural and non-structural elements, slab representation, soil-structure interaction, preexisting damage); the definition of the applied demand (type of analysis, loading patterns, ground motion selection, reference standards); and the diagnostic checks that reveal whether the model is correctly built (diagrams and data extraction, convergence problems, unexpected results, stability of the results against small changes). For each question an acceptable answer could be a thorough technical explanation, a deliberate simplification due to resource limitations or an honest acknowledgement of what remains unknown.