A correspondence is a rule that connects the elements of a set A (input set) with the elements of a set B (output set). A correspondence between the elements of the two sets A and B can be given in various ways, such as:

- With an arrow diagram. {e.x. FigA}
- By graph (set of ordered pairs) {e.x. FigB}
- Using type (if any) {e.x. y
^{2}+ 3x= -1} - With a price table

- With verbal-descriptive wording or symbolically {e.x. a tree grows 20 cm every year, so the height of the tree is related to its age using the function h, h=age ⸱ 20 }

**A function**is the special case of a pairing from set A to set B, where each element of set A is paired withelement of set B.*only one*- The set A is called the Definition Field
- The set of elements of B that are paired with elements of A is called the price range of the function.