The human brain may have an innate predisposition for numerical representation and knowledge. The parietal lobe is a key area for numerical processing, but also involved in verbal, spatial, and attentional functions. This study proposes a tripartite organization of number-related processes in the parietal lobe, based on fMRI and neuropsychological evidence. The horizontal segment of the intraparietal sulcus (HIPS) is a candidate for domain specificity, systematically activated during numerical tasks, independent of notation, and with increasing activation as quantity processing becomes more emphasized. This core quantity system, analogous to an internal "number line," can be supplemented by two other circuits: a left angular gyrus area supporting verbal number processing, and a bilateral posterior superior parietal system supporting attentional orientation on the mental number line. The HIPS is activated in various numerical tasks, suggesting a nonverbal representation of numerical quantity. It is active during mental arithmetic, number comparison, and specific numerical tasks. The HIPS shows greater activation for numbers than for other categories of words, and its activation is modulated by semantic parameters such as number magnitude and distance. It is also active in unconscious quantity processing, even when the subject is not aware of the number. Neuropsychological evidence supports the existence of a distinct semantic system for numerical quantities, with lesions in the left intraparietal sulcus leading to deficits in number processing. These deficits are often associated with Gerstmann's syndrome, which includes agraphia, finger agnosia, and left-right distinction difficulties. The left angular gyrus is involved in verbal number processing, particularly in tasks requiring verbal memory of arithmetic facts. It is active during exact calculation and is modulated by the verbal demands of the task. Neuropsychological evidence shows dissociations between arithmetic operations, with some patients impaired in multiplication and others in subtraction, suggesting a dissociation between verbal and quantity processing circuits. The posterior superior parietal lobule is involved in spatial attention and eye orienting, and is also active during numerical tasks. It is not specific to the number domain but plays a central role in visuospatial tasks. Neuropsychological evidence suggests that spatial attention contributes to numerical tasks, such as number comparison and bisection. Developmental dyscalculia is a common condition affecting 3–6% of children, and is often associated with left parietal disorganisation. It may involve a core conceptual deficit in the numerical domain, with symptoms such as dysgraphia, left-right disorientation, and finger agnosia. Subtypes of developmental dyscalculia may be distinguished based on differential impairments of quantity vs. language processing circuits.The human brain may have an innate predisposition for numerical representation and knowledge. The parietal lobe is a key area for numerical processing, but also involved in verbal, spatial, and attentional functions. This study proposes a tripartite organization of number-related processes in the parietal lobe, based on fMRI and neuropsychological evidence. The horizontal segment of the intraparietal sulcus (HIPS) is a candidate for domain specificity, systematically activated during numerical tasks, independent of notation, and with increasing activation as quantity processing becomes more emphasized. This core quantity system, analogous to an internal "number line," can be supplemented by two other circuits: a left angular gyrus area supporting verbal number processing, and a bilateral posterior superior parietal system supporting attentional orientation on the mental number line. The HIPS is activated in various numerical tasks, suggesting a nonverbal representation of numerical quantity. It is active during mental arithmetic, number comparison, and specific numerical tasks. The HIPS shows greater activation for numbers than for other categories of words, and its activation is modulated by semantic parameters such as number magnitude and distance. It is also active in unconscious quantity processing, even when the subject is not aware of the number. Neuropsychological evidence supports the existence of a distinct semantic system for numerical quantities, with lesions in the left intraparietal sulcus leading to deficits in number processing. These deficits are often associated with Gerstmann's syndrome, which includes agraphia, finger agnosia, and left-right distinction difficulties. The left angular gyrus is involved in verbal number processing, particularly in tasks requiring verbal memory of arithmetic facts. It is active during exact calculation and is modulated by the verbal demands of the task. Neuropsychological evidence shows dissociations between arithmetic operations, with some patients impaired in multiplication and others in subtraction, suggesting a dissociation between verbal and quantity processing circuits. The posterior superior parietal lobule is involved in spatial attention and eye orienting, and is also active during numerical tasks. It is not specific to the number domain but plays a central role in visuospatial tasks. Neuropsychological evidence suggests that spatial attention contributes to numerical tasks, such as number comparison and bisection. Developmental dyscalculia is a common condition affecting 3–6% of children, and is often associated with left parietal disorganisation. It may involve a core conceptual deficit in the numerical domain, with symptoms such as dysgraphia, left-right disorientation, and finger agnosia. Subtypes of developmental dyscalculia may be distinguished based on differential impairments of quantity vs. language processing circuits.