This is a handy little tip for when you are working with table borders in InDesign to hopefully make things just a little less fiddly. When you select the cells you want to work with the border tool appears in the info bar at the top of the page. This tool displays which borders on your selected cells will be affected by the changes you are about to make, any lines that are blue indicate borders that will be affected, any lines that are grey indicate borders that will not.
You can click on any border line to toggle its state one at a time, or you can toggle a horizontal and vertical outer border by clicking on an edge node, i.e. clicking on the bottom right node toggles the right and bottom border state (only if both are on or both are off).
But, here’s the tip – whatever combination of selected or unselected states the border lines are in, if you double click any outside line all outside borders become selected, double click again and they are all deselected. Likewise for the inner borders.
And there’s more, triple click on any border and all borders are selected, triple click again and they are all deselected!