Column Ruling

My tables at the moment have no border ruling on either rows or columns; which I think is the default setting.

I want to set a vertical rule between several, but not all columns. For example, between 3 and 4 (c and d) and 6 and 7 (f and g), but I’m not seeing en examples in the doc of FAQ.

Can you give me a hint or two, please?

Thanks!