[kjk]

Ok, I think I understand...a bit.
What I decided to do was to use H2 tags to separate my chapters, because I believe(?) Calibre correctly converts these to chapters.

So, I set my Chapter numbers/titles to H2, and that seems to work great-the iPhone sees these as chapters correctly.

So, my next question is...in Sigil I can assign Heading 1, Heading 2, Heading 3, quite easily using the Style dropdown menu. These correspond to H1, H2, H3 tags, right?

When I use H2 for my chapters, I get a BOLD, larger typeface than the rest of the document for the Chapter number/title that is within the H2/H2 tag

Where in the styles section of the document (the CSS) are H1, H2, H3 called out?
I don't see H2 in the CSS portion of the code, calling out text decoration: bold or anything.

What am I missing?

Here is the CSS:

<style type="text/css">
/*<![CDATA[*/
@namespace h "http://www.w3.org/1999/xhtml";
.calibre {
display: block;
font-size: 1em;
margin-bottom: 0;
margin-left: 5pt;
margin-right: 5pt;
margin-top: 0;
page-break-before: always;
text-align: justify
}
.calibre1 {
display: block;
margin-bottom: 1em;
margin-left: 0;
margin-right: 0;
margin-top: 1em
}
a {
color: inherit;
text-decoration: inherit;
cursor: default
}
a[href] {
color: blue;
text-decoration: underline;
cursor: pointer
}

@page {padding: 0pt; margin:0pt}
body { text-align: center; padding:0pt; margin: 0pt; }
div { margin: 0pt; padding: 0pt; }

@page { margin-bottom: 5.000000pt; margin-top: 5.000000pt; }

img.sgc-1 {height: 100%}

/*SG DO NOT MODIFY.
This style is used by Sigil.
It will be removed on export
along with the "sigilChapterBreak" HR tags. SG*/
hr.sigilChapterBreak {
border: none 0;
border-top: 3px double #c00;
height: 3px;
}

span.sgc-2 {font-style: italic;}
/*]]>*/
</style>

span.sgc-2 is italics, it seems (I used your Italics button to set some lines to italics, so I understand why font-style: italics)

The chapters are tagged as:

<h2>Chapter 2: The Wolf and the Lamb<br /></h2>

And show up as larger typefaces, in BOLD.

But I don't see h2 in the CSS? And i don't see any font style: bold setting?