Target

Source code

Explanation

Line 1: Without border=5pt, the grid line on the right will be truncated.¹

Lines 5–7: Using \multicolumn{1}{c} is the easiest way to centre only the heading that I know.

Lines 8–10: By trial, I discovered that putting p{3.5cm} at line 3 doesn’t help—the width of the middle column wouldn’t be the same as that of the other two columns. The positions of the |s and \multicolumn{1}{l}{Cell n} are also worked out by repeated experiments, so that the double vertical grid lines separating the cells are properly aligned.

Line 11: Actually, the double-rules can be made using \hline\hline. I don’t use this method because of the contents at the bottom of the table. See lines 25–26 for details.

Lines 25–26: \cline{2-2} is for a horizontal line above the second cell. If I use \hline\cline{2-2}, the two lines will coincide. Therefore, I copied the command \noalign{\smallskip} from the section “Space between rows” in the Wikibook for $\rm \LaTeX$.²

Lines 27–28: In order not to have grids at the corners of the last row, at left of the leading &, there’s no non-blank characters.

---