The photomask industry is experiencing a fundamental shift from Manhattan masks to curvilinear masks. In the recent lithography and mask technology conferences, there were many papers and talks on curvilinear masks, curvilinear optical proximity correction, curvilinear inverse lithography technology (ILT), curvilinear mask process correction, and curvilinear mask formats. Step by step, the photomask industry has started a transition from Manhattan to curvilinear, enabled by the adoption of the new multi-beam mask writers and the advent of practical full-chip curvilinear ILT. The benefits of curvilinear masks go much deeper than is immediately obvious. We will share our insights on why the mask world is moving toward curvilinear mask shapes. We will demonstrate that curvilinear masks are more reliably manufacturable. We will evaluate the benefits of curvilinear in terms of process window, mask rules, mask error enhancement factor, and mask variation.