One of Excel's features is the ability to format cells using different colors, fonts, number styles and border types. For example, you could separate columns with thin lines but use a thick border to group several columns together and make them visually distinctive from the rest of the spreadsheet. Doing this helps readers better read and retain the information that you're presenting.
Click the "Border" tool on the toolbar, and then click "Draw Borders" in the drop-down menu. The Border tool has an image of a nine-cell spreadsheet with a pencil overlaid on its upper-right corner. When your mouse is over the actual spreadsheet window, its pointer will be in the shape of a pencil, confirming that you are ready to draw borders.
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Click the "Border" tool again, and then click "Line Style" under the "Draw Borders" heading. A menu will fly-out horizontally, displaying a selection of line styles including three different solid lines of different thicknesses. To make a thicker border, click on the line style that is thicker than the one you are using.
Click on the actual cell border that you want to replace with the thicker line. Continue clicking borders until you are done changing the line thickness. If you want to draw a grid of the thicker border, hold down the "Control" key while you draw borders.
Click on the "Border" tool, and then click on "Draw Borders" again to turn draw borders mode off. Your mouse pointer will stop appearing as a pencil when you are in the spreadsheet.