Aim and background: The quad helix is well known for its ability to treat posterior crossbites in children with maxillary deficiency. A few authors have also used it for the treatment of unilateral crossbite due to unilateral maxillary deficiency by incorporating modifications in the appliance. The aim was to treat a unilateral crossbite using a newly modified quad helix. Case description: An 8-year-old female patient had presented with multiple missing teeth and a unilateral posterior crossbite due to an asymmetric maxillary arch. The amount of unilateral asymmetry was measured using a unique method over the cast. A modified quad helix was cemented to treat the unilateral posterior crossbite, and correction was obtained in 30 days. A Nance palatal arch was cemented which functioned as a space maintainer and a retention appliance. Conclusion: The posterior unilateral crossbite was treated successfully with the modified quad helix. Clinical significance: This article highlights a new modification of the quad helix for treating unilateral posterior crossbite and a new method of measuring maxillary asymmetry. The modification has been termed as a “bi helix” since it eliminates two helices from the classic quad helix design.