We examine the characteristics of linear astigmatism in a nonaxially symmetric optical system. The conventional view of interpreting linear astigmatism aberration using P and N focal surfaces is revisited. A perspective to explain and visualize the behavior of linear astigmatism is provided using the concept of field curves. Methods of controlling linear astigmatism are discussed, including a ray-tracing method using pupil imaging. Confocal mirror designs free from linear astigmatism are also discussed.