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Line spacing, or leading, is a more subtle question than is sometimes acknowledged. It relates not just to the absolute height of the characters (the bare minimum usually ensures that an 'h' and a 'y' – which have an ascender and descender, respectively – will not touch) but also to the width of the line, and indeed to the internal shape of the characters and the overall color of the page.

Unless the font involved has exceptionally long ascenders and descenders, more than any reputable text face does, merely not-touching is insufficient leading for optimum legibility. Hence the typesetter's minimum is generally 10/12 or 12/14 – a couple of points of leading. Many of us prefer more (10/13, 12/15 or 12/16) but we usually lose the fight for the extra paper. Three fewer lines a page in a 300-page book can mean another 20–25 pages, and the printing cost goes up.

Web text is usually set pretty tight: The Trebuchet and Arial samples shown on a previous page are essentially 12/14, the Verdana slightly more. One of the reasons I like Stone Sans is that it shows up naturally at 12/16; one of the reasons I hate the Apple Times and Helvetica is that they appear at 12/12.

Some of the variables, especially the subtle ones of color and the overall impression of a page, are hard to judge in advance at the best of times, and most of them are impossible to control on the web. In normal practice, typesetters and indeed designers fall back on well-grounded rules of thumb, the most relevant of which to the average web user is that wide lines require more leading. If they are too wide, of course, double columns are preferred; the adjustment comes in that in-between area.

Somewhere around four-and-a-half inches (27 picas in print terminology; 324 pixels at 72 dpi) is generally considered the optimum for book text, not much more and no less than about three. Sure, this is a narrow range, but then people's eyes fall into a narrow range. For a 28–35-pica column, which practical considerations may force you into, you add a little lead – not a huge amount, perhaps 5–20%. You are not making a statement here, you are trying to help the reader.

Basically, the wider the line, the more likely it is that a reader's eye will jump to the wrong new line unless the lines are well distinguished; and so the more leading the text needs.

Web text, which is usually about 20% bigger in size than standard book text (12-pt rather than 10-pt), might naturally run a little wider, say up to five-and-a-half inches (396 pixels at 72 dpi). Beyond that, you would like to increase the leading, but you can't.

Since, on the web, we cannot at present easily specify leading, we had better specify line width. And, in practice, we had better keep it short. These pages have a text column of 350 pixels, if everything goes right between me and you; perhaps a little narrow for 12/16, but as much as I'd like to see for 12/12 (not that I'd like to see that at all).

And of course, if you run into wide text, either because the page designer has made no effort to control it, or because they have asked for, say, 67% of the screen and you have a huge monitor – jump in there and fix it. Your comfort is the main thing; all these guidelines are just that.

The Electronic Telegraph site, which I like, rather cleverly keeps its left column (listing sections) constant – without using frames – while its right column (the first paragraph of, or the full, story) varies to screen width. Evidently it is designed, sensibly, for a 640-width screen. It's awkward at my full 800-pixel screen, because then I see about a 9-inch text width, and it would surely be weird at 1024; but if I narrow the window so it fits the fixed top logo that is so conveniently provided, the text width becomes seven inches or 42 picas, about500 pixels. This is still more than I prefer, but at 12/16 it's workable, and you do get a lot of words on the screen.

By the way, screens do vary in resolution – one study of a single department in Manchester found dot pitches running from 53 to 111 dpi – but what the heck, you have to assume something. Your objective reality may vary; your subjective one surely will.

There is a valid reason, I think, for double-spacing between paragraphs on screen, and not on high-resolution printed material. At least I have a theory about it, and offer this discussion as a simple experiment.
My theory is that the color of the page is affected by the thickness of stroke needed to generate a letter on screen. I haven't done any exhaustive study, but in the Times Roman examples above, the blown-up 12-pt sample has almost 30% more black pixels than the 60-pt. It seems obvious that a given line of text on screen has a greater percentage of black than the same line printed; thick strokes printed may be roughly the same as on screen, but thin ones are much thinner. This, of course, is also true of typewriters, which is where double-spacing came from.
When, therefore, you make a paragraph on screen, you get a block of black, from which the eye needs relief. A simple 12-pixel indent, as shown here, is enough to make a distinction at a distance, but not enough to make the overall page work, because the total proportion of 'ink' overwhelms it, whereas in a book the greater subtlety of the type, especially if properly leaded, allows us the luxury of making finer distinctions.
Single-spacing and indenting like this works tolerably for a paragraph or two, I think, but gets progressively less readable as the screen fills up, which is the conclusion I was hoping to reach. (Is this wish-fulfillment?) Certainly the most general point is this: For legibility, in any medium, there are numerous units to consider – the letter, naturally, but also the word (word space is an important variable), the line, the block of type and even the page itself.
If you take a perfectly good page of type and cut off the edges, that apparantly wasteful white space in the margin, it becomes less readable, and not just because there is nowhere to put your thumbs. I think the same is true if you make the margins too large, though this is rare. My guess is that there is an optimum ratio for text block to page, and that this will be different for a screen than for paper page. I suspect there are several reasons for this, including the need to train the eye to look in the right direction, the edge of the paper or window being a guidance point, and it is also a matter, I believe, of the ratio of ink to paper.
Or, of black pixel to white. OK, this is on gray – but that's another subject altogether, touched on briefly in the next page.