The letter M is no exception. The symbol for marriage, ·, has line symmetry but does not represent any real object. There are 14 letters with line symmetry: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z. They are shown in red in the diagram below. Symmetry is a very important concept in geometry because it tells us how objects are related to one another. Lines can be drawn through any letter that has line symmetry. For example, there is exactly one line through the middle of the M. This line divides the upper part of the letter from the lower part. Any property held by one part of the letter is also held by the other part. For example, if you measure a thing's length, you will get the same number whether you measure it from top to bottom or from bottom to top. There are two types of symmetry: spatial and mirror. Spatial symmetry occurs when something is interchangeable with its mirror image. So, the front and back of an object, the right and left sides, are identical.