Meaning of double + and - in Gitk's diff pane

Hello,

I regularly use gitk to get an overview. Great tool.
But sometimes it shows me in the diff pane lines preceeded
with two + or - signs or they are colored in blue or black
or printed in bold. I think this is true for merge commits
with conflicts.

The simple red - and green + lines are obvious, but is
anywhere documented what exactly the other fonts and colors
mean? I tried to "Ask The Audience" (which is Google), but
try searching for plus and minus! It's pointless.

Dirk