Compose to a Vertical Rhythm

“Space in typography is like time in music. It is infinitely divisible, but a few proportional intervals can be much more useful than a limitless choice of arbitrary quantities.” So says the typographer Robert Bringhurst, and just as regular use of time provides rhythm in music, so regular use of space provides rhythm in typography, and without rhythm the listener, or the reader, becomes disorientated and lost.

On the Web, vertical rhythm – the spacing and arrangement of text as the reader descends the page – is contributed to by three factors: font size, line height and margin or padding. All of these factors must calculated with care in order that the rhythm is maintained.

The basic unit of vertical space is line height. Establishing a suitable line height that can be applied to all text on the page, be it heading, body copy or sidenote, is the key to a solid dependable vertical rhythm, which will engage and guide the reader down the page. To see this in action, I’ve created an example with headings, footnotes and sidenotes.

Establishing a suitable line height

The easiest place to begin determining a basic line height unit is with the font size of the body copy. For the example I’ve chosen 12px. To ensure readability the body text will almost certainly need some leading, that is to say spacing between the lines. A line-height of 1.5em would give 6px spacing between the lines of body copy. This will create a total line height of 18px, which becomes our basic unit. Here’s the CSS to get us to this point:

body {
	font-size: 75%; 
}
html>body {
	font-size: 12px; 
}
p {
	line-height 1.5em; 
}

There are many ways to size text in CSS and the above approach provides and accessible method of achieving the pixel-precision solid typography requires. By way of explanation, the first font-size reduces the body text from the 16px default (common to most browsers and OS set-ups) down to the 12px we require. This rule is primarily there for Internet Explorer 6 and below on Windows: the percentage value means that the text will scale predictably should a user bump the text size up or down. The second font-size sets the text size specifically and is ignored by IE6, but used by Firefox, Safari, IE7, Opera and other modern browsers which allow users to resize text sized in pixels.

Spacing between paragraphs

With our rhythmic unit set at 18px we need to ensure that it is maintained throughout the body copy. A common place to lose the rhythm is the gaps set between margins. The default treatment by web browsers of paragraphs is to insert a top- and bottom-margin of 1em. In our case this would give a spacing between the paragraphs of 12px and hence throw the text out of rhythm. If the rhythm of the page is to be maintained, the spacing of paragraphs should be related to the basic line height unit. This is achieved simply by setting top- and bottom-margins equal to the line height.

In order that typographic integrity is maintained when text is resized by the user we must use ems for all our vertical measurements, including line-height, padding and margins.

p {
	font-size:1em;
	margin-top: 1.5em;
	margin-bottom: 1.5em; 
}

Browsers set margins on all block-level elements (such as headings, lists and blockquotes) so a way of ensuring that typographic attention is paid to all such elements is to reset the margins at the beginning of your style sheet. You could use a rule such as:

body,div,dl,dt,dd,ul,ol,li,h1,h2,h3,h4,h5,h6,pre,form,fieldset,p,blockquote,th,td {  
	margin:0; 
	padding:0; 
}

Alternatively you could look into using the Yahoo! UI Reset style sheet which removes most default styling, so providing a solid foundation upon which you can explicitly declare your design intentions.

Variations in text size

When there is a change in text size, perhaps with a heading or sidenotes, the differing text should also take up a multiple of the basic leading. This means that, in our example, every diversion from the basic text size should take up multiples of 18px. This can be accomplished by adjusting the line-height and margin accordingly, as described following.

Headings

Subheadings in the example page are set to 14px. In order that the height of each line is 18px, the line-height should be set to 18 ÷ 14 = 1.286. Similarly the margins above and below the heading must be adjusted to fit. The temptation is to set heading margins to a simple 1em, but in order to maintain the rhythm, the top and bottom margins should be set at 1.286em so that the spacing is equal to the full 18px unit.

h2 {
	font-size:1.1667em;
	line-height: 1.286em;
	margin-top: 1.286em;
	margin-bottom: 1.286em; 
}

One can also set asymmetrical margins for headings, provided the margins combine to be multiples of the basic line height. In our example, a top margin of 1½ lines is combined with a bottom margin of half a line as follows:

h2 {
	font-size:1.1667em;
	line-height: 1.286em;
	margin-top: 1.929em;
	margin-bottom: 0.643em; 
}

Also in our example, the main heading is given a text size of 18px, therefore the line-height has been set to 1em, as has the margin:

h1 {
	font-size:1.5em;
	line-height: 1em;
	margin-top: 0;
	margin-bottom: 1em; 
}

Sidenotes

Sidenotes (and other supplementary material) are often set at a smaller size to the basic text. To keep the rhythm, this smaller text should still line up with body copy, so a calculation similar to that for headings is required. In our example, the sidenotes are set at 10px and so their line-height must be increased to 18 ÷ 10 = 1.8.

.sidenote {
	font-size:0.8333em;
	line-height:1.8em; 
}

Borders

One additional point where vertical rhythm is often lost is with the introduction of horizontal borders. These effectively act as shims pushing the subsequent text downwards, so a two pixel horizontal border will throw out the vertical rhythm by two pixels. A way around this is to specify horizontal lines using background images or, as in our example, specify the width of the border in ems and adjust the padding to take up the slack.

The design of the footnote in our example requires a 1px horizontal border. The footnote contains 12px text, so 1px in ems is 1 ÷ 12 = 0.0833. I have added a margin of 1½ lines above the border (1.5 × 18 ÷ 12 = 2.5ems), so to maintain the rhythm the border + padding must equal a ½ (9px). We know the border is set to 1px, so the padding must be set to 8px. To specify this in ems we use the familiar calculation: 8 ÷ 12 = 0.667.

Hit me with your rhythm stick

Composing to a vertical rhythm helps engage and guide the reader down the page, but it takes typographic discipline to do so. It may seem like a lot of fiddly maths is involved (a few divisions and multiplications never hurt anyone) but good type setting is all about numbers, and it is this attention to detail which is the key to success.