spreadsheet on the same computer or network that is defined as shareable. These references contain the complete filename, such as: ='C:\Documents and Settings\Username\My spreadsheets\[main sheet]Sheet1!A1 In a spreadsheet, references to cells automatically update when new rows or columns are inserted or deleted. Care must be taken, however, when adding a row immediately before a set of column totals to ensure that the totals reflect the values of the additional rows—which they often do not. A circular reference occurs when the formula in one cell refers—directly, or indirectly through a chain of cell references—to another cell that refers back to the first cell. Many common errors cause circular references. However, some valid techniques use circular references. These techniques, after many spreadsheet recalculations, (usually) converge on the correct values for those cells. Cell ranges Likewise, instead of using a named range of cells, a range reference can be used. Reference to a range of cells is typical of the form (A1:A6), which specifies all the cells in the range A1 through to A6. A formula such as "=SUM(A1:A6)" would add all the cells specified and put the result in the cell containing the formula itself. Sheets In the earliest spreadsheets, cells were a simple two-dimensional grid. Over time, the model has expanded to include a third dimension, and in some cases a series of named grids, called sheets. The most advanced examples allow inversion and rotation operations which can slice and project the data set in various ways. Formulas A formula identifies the calculation needed to place the result in the cell it is contained within. A cell containing a formula, therefore, has two display components; the formula itself and the resulting value. The formula is normally only shown when the cell is selected by "clicking" the mouse over a particular cell; otherwise, it contains the result of the calculation. A formula assigns values to a cell or range of cells, and typically has the format: where the expression consists of: values, such as 2, 9.14 or 6.67E-11; references to other cells, such as, e.g., A1 for a single cell or B1:B3 for a range; arithmetic operators, such as +, -, *, /, and others; relational operators, such as >=, <, and others; and, functions, such as SUM(), TAN(), and many others. When a cell contains a formula, it often contains references to other cells. Such a cell reference is a type