Why margins measured in em units?

[radius]

I've wondered for a while now why many books I have seem to use em units to specify the page margins instead of pixel units.

This is very annoying with professionally produced books from traditional publishing houses. To me, it looks like they try to reproduce the look of a regular paperback novel on the screen of the device (a PRS-505 in my case). This means significant amount of whitespace around the border of the page and fairly small text.

Unfortunately, the pixel density of e-ink screens isn't all that high yet, making text that small only marginally readable. So in many lighting conditions I have to magnify it to one or even two sizes larger. Since the margins are proportional to font height that means the originally large margins become even wider and at the largest font size I only get three or four words on a line, even though the text isn't very large!

Wouldn't it be better to use a fixed number of pixels for the page margins? Then they margins might be wider than I like, but at least they wouldn't grow even more when the text is magnified.

[JSWolf]

I have never seen the margins get larger when you increase the text size. Can give us some examples of this happening and where you got these ebooks?

[HarryT]

I use "em" units to specify indentation in poetry, Jon, because for that I think it's right that the indentation should scale proportionally with the font size.

[JSWolf]

I can understand indents shifting. But why would you want the margins shifting? In fact, just do away with the margins and you'll have a better reading experience (IMHO).

[HarryT]

I agree with you that you probably wouldn't want the margins to scale with the font.

[tompe]

Originally Posted by JSWolf

I can understand indents shifting. But why would you want the margins shifting? In fact, just do away with the margins and you'll have a better reading experience (IMHO).

I think you might want it if you also specify the text width in em. Then the font choice will decide the size of the paper. Of course you do not want it for text on an E-book.

1 em is also the width of the letter m in the chosen font. It is not the height.

[JSWolf]

If I make eBooks that have a margin, I specify a fixed margin. I do not mind em used for indent if it's not too large. I've noticed that a lot of Mobipocket eBooks use 5% and that is huge for text indents.

Em unit is the following. The unit is the width of the widest character in the font that you are using. Typically it is an M or W. The two used Em items are the Em Space and the Em dash. Both are the width of an M. Halft of the Em is called an En space or dash.

En Dashes are used for dashed in numbers like your telephone number. The Em dash is used to break a sentance. You can type this (--) in word and it will auto correct to am Em dash.

I have worked in the Printing indistry since 1994 and I have never come across anyone measuring the margin in Em spaces. I have seen millimeters, centimeters, points (72 per inch), Picas (12 points to a pica), and Inches used. Typically Picas are used for line lengths. A typography rule of thumb is that the line legth should be about two times (in Picas) the font size (points). Example. If you are using 12 point type, your line lengths should be around 24 Picas. This makes it easlier to read. Long lines of type are harder for the eye to follow.

Okay . . . again, I have never seen anyone use Em spaces to set up a margin. I would use an Em space for an endent rather than using the tab.

[mtravellerh]

Jon is right. Margins are best defined in percentage. Keep it small 2-5%. Some prefer no margins at all.

[radius]

Hmmm... The book I most recently saw this in is the novel Fletch from Random House which I borrowed from the library. I already returned it so I can't tell you any more than that.

If you are looking for concrete examples, I will look through my library when I have time. But my impression is that this is very common.

Edit:

I got curious so I re-borrowed Fletch and used Inept on it.

The "regular" paragraphs in the text are of class "indent" and in the stylsheet, p.indent just says indent by 1em and text-align:justify.

However, the body specifies 1em left margin and 2em right margin...

[radius]

Originally Posted by JSWolf

I can understand indents shifting. But why would you want the margins shifting? In fact, just do away with the margins and you'll have a better reading experience (IMHO).

I think a small amount of margin is needed for a couple of reasons.

1) On some readers, the surface of the screen may be slightly lower than the surface of the outer casing, so you will get shadowing on the edges of the screen, so a couple of millimetres of margin at least would be nice.

2) Adobe's ePub renderer on Sony devices prints page numbers in the right hand margin so you need to add some extra space to avoid overlapping them.

[Elfwreck]

Originally Posted by Linus

Em unit is the following. The unit is the width of the widest character in the font that you are using. Typically it is an M or W. The two used Em items are the Em Space and the Em dash. Both are the width of an M. Halft of the Em is called an En space or dash.

Wikipedia says that's not entirely correct; an em unit = point size. (So 1 em for a 16-pt font is 16 pts.) It also says that "width of an M" is the former definition, because with printing presses they were the same, but with modern fonts they don't always match up.

Okay . . . again, I have never seen anyone use Em spaces to set up a margin. I would use an Em space for an endent rather than using the tab.

Apparently, it's suggested for webpages, where what the viewer sees depends as much on what kind of filters they're using (browser, plugins, converters, etc.) as the raw html.

I suppose some ebook publishers have decided that, since ebooks are a "scrollable" medium, they should be treated exactly like webpages, and get em-based margins.

Interesting. I guess I learned a bit about the web coding today. Odd: Em = Point size. Somehting to add to my head.