Lines can be straight or curved but rule lines are typically straight horizontal, vertical, or diagonal lines although they may meet to form sharp or rounded corners.
Rules are normally expressed in point sizes although some programs use inches by default. A hairline rule is the smallest size and is usually about one-fourth of a point, although this can vary from one program to another. Most page layout programs come with several preset or one-click width rules usually from hairline to 8, 16, or 24 points. However, you can customize rules for other sizes including partial sizes such as 1.5 or 2.6 points if you use specific sizes on a regular basis.
Solid rules aren't the only possibility. Some software programs offer a wide variety of pre-set rules styles or you can create your own. Make rules from round, square, or diamond-shaped dots. Mix dots and dashes in a pattern. Combine solid or non-solid rules in different sizes.