CSS glitch: borders with table rows

I am trying to put borders on tables rows using CSS, something like
this:

<tr class="rowlines"></tr>

where

tr.rowlines {
                border-bottom 2px solid gray;
                }

This has no effect at all in the browsers I've tried. I can put the
border in the <td> elements and it shows up, but I get the gap
between columns.

Is this something that can be done?

[BTW, the preview is putting a space in the word "botto m," but it
isn't there]