This work extends to Mesoamerica researches on archaic geometry previously performed on architectures of the “Old World”. CAD (Computer Aided Design ) tools are used to fit theoretical forms to spatial segments on archaeological plans. The analyses provide quantitative information on the forms, on the numbers involved and on the units of length used. Everywhere, numbers appear mainly to be multiples of 9, 10 and 13, rather than pertaining to Squaring Triads, although these are nonetheless attested in all regions. Three units of length are found: a “Teotihuacán unit” t ≈ 0.58 m, a “Monte Albán unit” ma ≈ 0.50 m and a “Chichen Itza unit” ci ≈ 0.66 m. Units t and ma clearly appear to be linked to the distances between the main monuments, associated with the Tzolkin calendrical numbers 13-20 at Teotihuacán and with the Haab calendrical numbers 18-20 at Monte Albán.