The border-spacing property controls the amount of space between adjacent cells in a table when the table uses the separated borders model. It creates a visible gap between the outer edges of neighboring cell borders so that the table's background (or whatever sits behind the cells) is exposed in that gap rather than the cell backgrounds being contiguous. This property only has an effect when the table is not using collapsed borders - see border-collapse for how the two border models differ.

At a conceptual level, border-spacing defines independent horizontal and vertical gaps between the boxes that represent table cells, allowing different spacing in each direction if desired. Those gaps are applied outside each cell’s own border, so they do not replace cell borders or internal padding; instead they add extra separation and therefore contribute to the table’s overall width and height. Because the gaps sit between cell borders, cell backgrounds do not fill the spacing - the table background (or underlying content) shows through.

In practice, designers use border-spacing to create breathing room between cells without increasing the internal padding of cells or adjusting individual cell margins. It’s a convenient way to control the visual rhythm of a table - for example to make grid data easier to scan - while keeping borders and cell contents unchanged. Keep in mind that using spacing influences layout calculations, so when precise alignment or fixed table sizing is required you may need to combine it thoughtfully with other layout choices or use alternative techniques.