Hidden lines show edges and contours of important features that are obscured by the geometry of the part. They are drawn as short dashes that are equally spaced. Hidden lines will always begin and end with a dash. Sometimes the length of the dash will need to be adjusted to show a break, but the overall appearance of the dashes should be consistent on the drawing.
Hidden lines are not absolutely necessary for every feature and can be omitted if it makes the drawing more clear. If you do omit hidden lines, it is a good idea to add a note indicating this.
Hidden lines are thin.