For incisors to erupt in normal alignment, there is an obligate space requirement in the anterior part of both the arches which is termed as incisor liability. The total sum of the mesiodistal width of four permanent incisors is larger than that of primary incisors by 7.6 mm in maxilla and 6 mm in mandible (Wayne). This obligate space is provided by:Interdental physiologic spacing in the primary incisor region: 4 mm in maxillary arch; 3 mm in mandibular arch. Increase in the intercanine arch width: Significant amount of growth occurs with the eruption of incisors and canines. Increase in the anterior length of the dental arches: Permanent incisors erupt labial to the primary incisors to obtain an added space of 2-3 mm. Change in inclination of permanent incisors: Primary teeth are upright, but permanent teeth incline to the labial surface, thus decreasing the inter-incisal angle from about 150o in the deciduous dentition to 123o in the permanent dentition. This increases the arch perimeter.Textbook of Orthodontics, Gurkeerat Singh, 2nd ed Page 45-46