There are three rules at work here:

p.One {

color: red;

}

p.Two {

font-weight: bold;

}

p.One.Two {

font-style: italic;

}

This paragraph has class="One", so it should have red, but otherwise plain, text.

This paragraph has class="Two", so it should have bold, black text. But because IE6 interprets the third rule above as though it were just p.Two, it will also be (incorrectly) italic in IE6.

This paragraph has class="One Two", so it should have bold, italic, red text.