When designing with type, the use of numbers can take a layout from good to great. I'll detail the four main styles below, what they look like, and how to use them to the best of their ability. I'll add a few tidbits of related information as we go.
Numbers, actually, can be anything, 10, 200, 0.0125, whatever. So, when typographers talk about any one of those characters that make up numbers, we call them figures. 10 is a number, but 1 and 0 are figures.
Historical interlude: Ever notice why figures don't always look that similar to the Latin script (what we use to write English)? That's because they are of Arabic origin. The Romans used Roman numerals, of course, up until about the 12th Century (1300 A.D.) when the Arabic style took over Europe.
There are four basic styles. Let's dive in.
1. Tabular Lining
Best for: Tables & Columns
The most common figures found in fonts are tabular lining figures. Tabular meaning for tables, of figures that are all the same advance width. Lining meaning the figures align horizontally with each other, usually around the height of the capitals. Think of an Excel spreadsheet, or a receipt. When the figures are aligned, it makes it easier to read the table and scan for information. Making something easy to read is one of the typographer's main goals.
Some typefaces have figures a little bit shorter than the caps to balance the color and weight with the rest of the typeface. In some cases, if the figures were as heavy (stroke width) as the caps, they would stand out as being too dark. It's an optical adjustment thing, and a stylistic option for the type designer. See Helvetica, as an example, of figures that are a little shorter than the caps.
The stand-out characteristic of tabular lining figures is usually a one with a lot of white space on both sides, sometimes compensated for with a flag and foot serifs to try to fill more of the white space.
This is a similar concept to monospaced fonts, where all of the glyphs are the same width, as seen in 'typewriter' designs.
2. Proportional Lining
Best for: Headlines, some text
Proportional lining is probably the second most common style. These are lining again, as above, similar to cap height. Instead of being tabular, they are proportionally spaced. The horizontal advance width is proportional to the shape of the figure. This means the one will be narrower than the five, for example. The proportional spacing allows for a more even appearance, a balance of black and white.
These can be preferable in text over tabular lining figures, and can improve headlines dramatically.
3. Proportional Old Style
Best for: Text
Proportional old style figures are designed for text. They bounce up and down on the baseline, blending in with the lowercase, proportionally spaced for even typographic color. For a period of time, this was the way that figures were written. Later on, maybe 19th Century or so, we saw the introduction of lining figures, so the 'old style' moniker is referring to the dominant style before lining figures became popular. Everything is relative.
Old style figures might not align with the lowercase. It's commonly a bit higher, like small caps. Some designs are famous for using proportional old style figures by default, like Georgia and Adelle. This style can be used for stylistic purposes as well. For example, the headlines on TheVerge.com have old style figures, which make the tech news site feel more sophisticated.
4. Tabular Old Style
Best for: Tables & Columns, in that Old Style flavor
This is the odd man out. As the name describes, they are tabular yet with the old style shapes. This makes sense from a logical point of view, filling out the set: Lining in tabular and proportional varieties, and old style in tabular and proportional. I have yet to see these used in the wild (outside of the font design world). If you are setting a table that requires that old style mood, look no further.
Some new designs coming out are including small cap sized figures as well. Again, I haven't seen these in the wild, but I suppose they could be useful. A broader palette of styles to work with. But, pairing small caps with proportional old style is recommended as well.
It can be hard to tell if your favorite font has these options. First, you could check the menus shown below, trial and error. Some font packaging might give you a clue as well, something with 'Pro' in the name might have these extras. You could contact the type foundry or designer directly. Many font vendors, like MyFonts.com, will show details about the font, including OpenType features. So, have a look, either before or after you purchase a license.
How-To Use Figures With Adobe Applications
The OpenType options in the Adobe apps vary. They could sync that up better, and they are a little buried. This is what separates an amateur from a professional—details—and fine typography is all about details.
The menus controlling OpenType features haven't changed much in a long time, so these directions should be applicable to a few legacy versions. Of course, you can always check with Adobe's documentation, when in doubt. The easy part is flipping the switches in the app, the hard part is learning when and where to use the different styles.
Start at the OpenType palette, and select the pop-up menu labeled Figure. Easy!
Start at the Character palette, click on the little hamburger icon menu (three horizontal stripes), then OpenType, then at the bottom of the list is the figure options. There is also OpenType options under the powerful Character Style and Paragraph Style (under Type menu).
That's it for now. Let me know if you want exposure on other features or styles.
Updated 1-24-14. Fixed a few typos.