What type of lines need to be attached to the cylinder to check for straightness?

To check for straightness on a cylinder, attaching two lines is essential because it allows for a more accurate assessment of the cylinder's alignment and potential deviations. When two lines are used, they can be positioned at opposite ends of the cylinder, creating a reference point that highlights any irregularities along the length of the cylinder. This method effectively identifies bowing or bending by providing a straightforward visual comparison, which is crucial in ensuring that the cylinder is functioning properly and maintaining safety standards.

Utilizing just one line would not provide a reliable measurement, as it could lead to misinterpretations without a second reference point to compare against. Similarly, employing three or four lines would introduce unnecessary complexity, making the process more cumbersome without significantly enhancing the accuracy of the straightness check. Thus, using two lines strikes the right balance between simplicity and effectiveness in inspecting a cylinder's straightness.