To determine the main mandibular dental arch forms, 278 dental casts of untreated French adults with normal occlusions were examined. Six measurements of the mandibular dental arches were performed, and five independent ratios were determined. By following the k-means clustering method on the basis of these ratios and the use of polynomial functions of the sixth degree, five mandibular dental arch forms were defined, and an arch guide was developed. The proposed forms are relatively narrow in comparison with previous studies. No significant differences in their distributions were seen between male and female subjects. However, the dental arches of the women have smaller dimensions.