Word order, case assignment, and agreement for gender and number are realised with remarkable complexity in the Arabic numeral system. This paper examines the internal morphological structure of simplex, compound, and complex numerals. We identify a recurrent pattern found both inside complex numerals and in the structural relations between numeral and the nouns they quantify. The structures uncovered then allow for more principled accounts of the superficial morphosyntactic complexities. The analysis suggests that DP contains a single Num head, but that Num can express both additive and multiplicative arithmetic operations.