Linguistic irregularities of number naming systems, such as inverted number words (e.g., the German number word for 32 translates to “two-and-thirty”), affect the processing of Arabic numerals. Recently, it has been claimed that there is even a number word syntax representation, i.e. a specific system dedicated to representing the syntax of number words in the human brain. However, to date, studies have mainly focused on the effects of inversion on performance in simple and calculation-based processing of numerical information. The effects of number syntax in non-inverted but still intransparent number word systems such as French have not yet been explored to a similar extent, especially their neuro-cognitive correlates during development, although the French number word system might be particularly suited for studying the neural correlates of a specific number word syntax representation. In the proposed project, we aim to investigate the development of the unit-decade-compatibility effect (UDCE) in number magnitude comparison in children (aged 6.5-7.5 and 9.5-10.5 years) and adults using combined behavioral and neuroimaging (fMRI) data, because the UDCE known to be sensitive to linguistic irregularities. This will allow us to triangulate the neuro-cognitive correlates of number word syntax processing and their developmental trajectories. Thereby, the proposed project will contribute to the knowledge of how number word syntax is processed in the developing brain and pave the way for improved remediation or education concepts.