Tom,
The manual states that only 2-Rank Infantry suffer a loss of firepower for being in shortened line and benefit from greater firepower in extended line. As far as I am aware that would only include British and KGL infantry in the Waterloo game, unless I'm forgetting some others. Most other nations have 3-rank line formation. So if this is the case then the only benefit of going into extended line for 3 rank infantry is that you can cover a longer frontage with your battalions. Of course this would make them more vulnerable to melee attack though. Someone will correct me if I have this all wrong.
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