For example, from first grade to third grade, the block diagram is consistently used for explaining place value. The block is not limited to explaining the base ten systems and counting by ones but is also used for counting by multiple bases for multiplication. From second grade to sixth grade, the tape diagram in multiplication is combined with the number line to represent proportionality. This is called the proportional number line. These limited representations are formally and consistently used to enable children to extend four operations and ideas by themselves.