Inclusion Criteria:  prepubertal phase of development (cervical stage CS1 or 2 in cervical vertebral maturation or corresponding stage of third Finger middle phalanx maturation index). early or intermediate mixed dentition stage with fully erupted upper and lower first permanent molars. presence of the second upper deciduous molars available as an anchoring tooth. The second deciduous molar was considered available as anchoring tooth when the root had the same length as the clinical crown at the radiographic examination (Quinzi V, Federici Canova F, Rizzo FA, Marzo G, Rosa M, Primozic J. Factors related to maxillary expander loss due to anchoring deciduous molars exfoliation during treatment in the mixed dentition phase. Eur J Orthod. 2021 Jun 8;43(3):332-337. doi: 10.1093/ejo/cjaa061) Posterior transverse interarch discrepancy (PTID) of at least 3 mm. PTID was calculated on dental casts with a caliper as the difference between the maxillary intermolar width (distance between the central fossae of right and left permanent first maxillary molars) and the mandibular intermolar width (distance between the tips of the distobuccal cusps of right and left permanent first mandibular molars) (Tollaro, I., Baccetti, T., Franchi, L. and Tanasescu, C.D. 1996 Role of posterior transverse interarch discrepancy in Class II, Division 1 malocclusion during the mixed dentition phase. AJODO, 110, 417-422).  Exclusion Criteria:  age older than 12 years, pubertal or postpubertal stage of development (CS 3-6), late deciduous or late mixed dentition, 4. agenesis of upper second premolars (assessed on initial panoramic radiograph), Class III malocclusion, cleft lip and/or palate and craniofacial syndromes