When sourcing custom glass bottles, buyers often focus on capacity, bottle shape, color and appearance. However, one of the most important technical details is often overlooked: the glass bottle neck finish. The neck finish determines how the bottle connects with the closure. Depending on the application, a bottle may use a screw cap, ROPP cap, cork, T-top stopper or another type of closure. The bottle neck and closure must be designed and specified as a compatible system. A mismatch between the bottle neck and closure can result in leakage, poor sealing, difficult capping, loose closures or problems on an automated filling line. This is particularly important for B2B projects involving custom glass bottles, where changing the neck finish after mold development may require engineering adjustments or mold modifications. In this guide, we explain the most common glass bottle neck finishes, including GPI, ROPP, screw, cork and T-top systems. We also explain important neck dimensions, thread specifications and closure compatibility so you can provide the right information when sourcing glass bottles in bulk.