Whenever you ->pack a widget (and frames are just another widget that you happen to be able to put things into), the space gets divided into two pieces. One gets the widget and the other remains available for the next pack operation.

If you pack(-side => 'top') or pack(-side => 'bottom'), the container is split by a horizontal line and you take whichever part you claim. 'left' and 'right' split with a vertical line.

ASCII illustration follows:

+----------------------+   +-----------+----------+
| pack(-side => 'top') |   |           |          |
|                      |   |  left     |   right  |
+----------------------+   |           |          |
|                      |   |           |          |
|   bottom             |   |           |          |
+----------------------+   +-----------+----------+

Note that each pack further splits the space in two. You can not use pack to do:

+----+---------+
|    |         |
|    +----+----+
|    |    |    |
+----+----+    |
|         |    |
+---------+----+

If you need to do something like that, you have to use a different mechanism, and I don't know that well enough to speak off the top of my head.

Another way to look at it is:

You start with a rectangular space. You use pack to grab a rectangular chunk that touches three sides but not the fourth, into which you pack your widget. The remaining rectangular space is where the next pack goes. If the widget you pack is a container like a Frame, you have a new space to start with.

"It's rectangles all the way down..."

yours,
Michael