Assessment of skeletal maturity is crucial for managing growth related problems. Tanner and Whitehouse (TW) hand and wrist bone age assessment is an accurate method; however, it is complex and labor-intensive. Several simplified methods derived from the TW method were proposed, and each had its own character. The purpose of this study is to explore the relationship between these methods for accurate usage.Between 2018 and 2019, a cross-sectional study was performed with consecutive left hand and wrist x-rays obtained from a pediatric orthopedic clinic. Bone age assessments included the distal radius and ulna (DRU) classification, Sanders staging (S), thumb ossification composite index (T), and TW method. Somers delta correlation was conducted to determine the interchangeability of these stages. The mean bone age and standard deviation (SD) of each subgrade were compared and analyzed.Totally 103 films (92 girls) were analyzed with mean age of 12.1 years (range: 8.0-17.9 years). The radius (R) stages had good correlation with S, T, and U stages with a very high Somers delta correlation (P < .05). R5 had relatively large SD (1.5) and referred to T2 and T3; R6 and R7 had the smallest SD (0.3) with reference to T4 or S2; R8 referred to T5 or S3, S4, S5; R9 referred to S6 and S7.The internal relationship between the DRU and digital stages system was well proven. We also provided a simple and accurate way to assess the bone age by combination of some subgrades with smaller SD: 10y-proximal thumb covered without sesamoid (T2); 10.5y-sesamoid just appearing (T3); 11y-distal radial physis just covered (medial double joint line, R6); 11.5y-medial capping of distal radial physis (R7); 12y-bilateral capping of distal radial physis (R8) / phalangeal capping without fusion (S3); 12.5y-distal phalangeal physes start to fuse (S4); 13y-distal phalangeal physes fused (S5); 13.5y-proximal phalangeal physes start to fuse (S6); 14.5y-proximal phalangeal physes fused (S7); 15y-distal radial physis almost fused (R10).Level of Evidence: Diagnostic study, level III.