Background: Twin-block appliance is a widely used myofunctional appliance to correct a skeletal discrepancy in growing class II individuals. In patients with deep anterior bite and horizontal growth pattern, the posterior maxillary block has to be trimmed for the active eruption of lower molars to flatten the curve of spee. The upper block is trimmed 1–2 mm every appointment to encourage lower molar eruption. By keeping the minimal clearance between the lower molar and upper block the tongue is prevented from spreading laterally between the teeth. This might prevent the eruption of mandibular posterior teeth. We have modified the twin block appliance by incorporating two colors alternatively for the maxillary block. This modification acts as a guide for controlled trimming of the upper block. Conclusion: It is a simple and effective way to prevent the development of lateral open bite during the active phase of twin block therapy. Clinical significance: Prevents excessive trimming of the blocks hence reduces the risk of developing posterior open bite due to tongue spreading laterally between the teeth.