“Not enough room between lines” is listed as the number one mistake on this list of common typographic errors. Many of the others are aimed at books and web pages rather than large posters, so keep that in mind when scanning the rest of the list – particularly when you see, “Large body copy” listed as a mistake.
The space between lines in known in the typography business as leading (rhymes with “sledding”). What should you consider when looking at leading on a poster?
First, look at how long your lines of text are. The lines are often quite long on posters, even though this can be mitigated by the reader being some distance away. When you reach the end of the line you’re reading, you have to scan back and down to the next line. The further you have to scan back to the beginning of the next line, the more likely it is that you will lose your place. Increasing the leading helps make each line distinct in long text.
Second, look at the typeface you have. If you have a typeface with very long ascenders (the part that is taller than most lowercase letters, like the pointing up bits on l, k, and t) and descenders (the dropping down bits on letters like p, g, and y), increase the leading. You don’t want your words colliding!
Lowercase letters can pose more a subtle problem. If you have a typeface where the lower case letters are very close to the upper case letters in height (known as a large x-height), again, you’ll want to increase the space between the lines. Letters with a large x-height tend to form swaths of gray if placed too close together.
Incidentally, one more reason not to use PowerPoint to make a poster is that it tends to automatically squish the leading down to make text fit. If you’re not paying attention, your single-spaced text will change without warning to 0.9 spaced text or smaller.