Reply To: Number Lines

1. Bold line goes from the solid dot to the right arrowhead (not the whole line)
2. Bold line from left arrowhead to the empty dot
3. Bold line starts at the solid dot and goes to the right arrowhead

A bold arrow head generally has meaning. Because all of the arrowheads, including those at the end of a not-bold line, look the same, I would not use the double arrowhead (bold) on any of them.