References: relative, absolute, and mixed ($)

When you copy a formula to another cell, Excel shifts the addresses in it by default. The reference A1 is called relative: copy the formula one row down and A1 becomes A2; copy it one column to the right and it becomes B1. That's handy when you need to apply the same operation to every row. But sometimes one of the addresses must stay put — for example, the exchange rate in cell D1 that every price has to be multiplied by. Then you use the absolute reference $D$1: a $ sign before the letter locks the column, and a $ sign before the number locks the row. However you copy it, $D$1 won't shift. Mixed references lock only one of the two coordinates. A$1 — the row is locked (copied down, it stays in row 1, but the column can still shift). $A1 — the column is locked (copied to the right, it stays in column A, but the row can still change). The rule: the $ sign locks whatever it stands in front of — before the letter it locks the column, before the number, the row. A typical beginner's mistake: forgetting to put $ on the cell with the exchange rate or the sales tax rate. When the formula is copied, it starts referring to empty or random cells, and the result is wrong.