Occasionally, you are required to perform operations where the element of one side is matched with several elements on the other side of the operation. When this happens, you are required to provide Prometheus with the means to interpret such operations. If the higher cardinality is on the left-hand side of the operation, you can use the group_left modifier after either on or ignoring; if it's on the right-hand side, then group_right should be applied. The group_left operation is commonly used for its ability to copy labels over from the right side of the expression, as will be seen on some practical examples later in this chapter.