The "$", read "string" in the days of Basic programming, tells excel to hold
the following row or column value as a constant when the formula is copied to
a new cell. The $ notation can be applied to either the row or column (or
none or both). Function key F4 will quickly toggle between all of the
options when editing the cell. For instance, in the example of D$7, if you
were to copy this formula to row 8, it would continue to use the value in D7.
If you were to copy this formula to column E however, it would use the value
in E7.
