If you are using a single color line, you can try to force a size to it. If - for example - your GIF file name is red.gif, try this: img width="2" height="1000" src="red.gif" Put the above sentence in <> brackets
I have coded a verticle line with a nested table. You can see the code here:
http://www.tloffman.com/vertline.txt
I left the borders set to 1 so you can see what the tables look like. To get rid of the borders set border=0 in both the outer and inner table. To vary the position of the line change the td width=25% to any percentage you like. You can also change the color of the line by changing the bgcolor="anycolor". Play with this code and you might want to use some version of it. Nested tables are very handy, but you should always include comments so you can see what's going on and not get lost.
