When Org exports tables, it does so by default without vertical lines because that is visually more satisfying in general. Occasionally however, vertical lines can be useful to structure a table into groups of columns, much like horizontal lines can do for groups of rows. In order to specify column groups, you can use a special row where the first field contains only ‘/’. The further fields can either contain ‘<’ to indicate that this column should start a group, ‘>’ to indicate the end of a column, or ‘<>’ to make a column a group of its own. Boundaries between column groups will upon export be marked with vertical lines. Here is an example:
| N | N^2 | N^3 | N^4 | sqrt(n) | sqrt[4](N) |
|---+-----+-----+-----+---------+------------|
| / | < | | > | < | > |
| 1 | 1 | 1 | 1 | 1 | 1 |
| 2 | 4 | 8 | 16 | 1.4142 | 1.1892 |
| 3 | 9 | 27 | 81 | 1.7321 | 1.3161 |
|---+-----+-----+-----+---------+------------|
#+TBLFM: $2=$1^2::$3=$1^3::$4=$1^4::$5=sqrt($1)::$6=sqrt(sqrt(($1)))
It is also sufficient to just insert the column group starters after every vertical line you would like to have:
| N | N^2 | N^3 | N^4 | sqrt(n) | sqrt[4](N) |
|----+-----+-----+-----+---------+------------|
| / | < | | | < | |