Ships would normally sail one behind the other "in line" (hence the term "lineships").
<img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

" border="0" alt="sailr.gif" /> <img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

" border="0" alt="sailr.gif" /> <img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

" border="0" alt="sailr.gif" /> <img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

" border="0" alt="sailr.gif" /> <img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

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They would the pass parallel to each other blasting broadsides as they did, and that's how they fought.
Crossing the "T" meant that one fleet would cross the enemy line at the head of it's line (which put it at right angles to the opposing fleet).
<img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

" border="0" alt="sailr.gif" /> ^ Direction of sail
<img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

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<img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

" border="0" alt="sailr.gif" /> <img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

" border="0" alt="sailr.gif" /> <img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

" border="0" alt="sailr.gif" /> <img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

" border="0" alt="sailr.gif" /> <img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

" border="0" alt="sailr.gif" /> <img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

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<img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

" border="0" alt="sailr.gif" /> < Direction of sail
<img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

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<img src="style_emoticons/<#EMO_DIR#>/sailr.gif" style="vertical-align:middle" emoid="

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They could then fire their broadsides down the length of the other fleet, but the other fleet could only return fire from the bow guns of the lead ship.
The line under attack could only avoid this by each ship making a right or left turn (depending on the wind) and would hence break their formation and leave themselves vulnerable to further broadsides while making the maneuver till they could bring their guns to bear, as their own ships would still be crossing in front of them while they were all trying to turn.