DBB was originally manufactured and proposed for checking the accuracy of NC machine tools. The device can also be used to study the relations between motion error of trace and the machine motion error for the purpose of error compensation. The nominal angle obtained through rotation feed rate and sampling time was adopted instead of actual angle which cannot be measured by the device. This project examined in details the effect of the angle deviation problem caused by non-perpendicularity between the movements in axis X and axis Y. The analysis and calculation verified that a squareness deviation would lead to more than 3 times error at some special points, decreasing the accuracy of NC machines for error compensation. For this reason, a device called as Two-Dimensional Ball Bar is proposed based on the DBB.