The human eye is very receptive to differences in brightness over a text body. One can therefore differentiate between types of emphasis according to whether the emphasis changes the "blackness" of text.
A means of emphasis that does not have much effect on "blackness" is printing in italics, where the vertical orientation of all letters is tilted to the right. With this technique, words can be highlighted without making them "stick out" much from the rest of the text. Traditionally, this is used for marking passages that have a different context, such as words from foreign languages (for example, in German: Fremdsprachen), book titles, etc.
By contrast, boldface makes text darker than its surroundings. With this technique, the emphasized text strongly stands out from the rest; it should therefore be used to highlight certain keywords when it is presumed that the reader might be looking for certain passages while allowing him to skim over those that deal with other topics. For example, printed dictionaries and other wordbooks would use boldface for their keywords; Wikipedia follows this convention when the keyword that an article was written for is marked at the top.
With both italics and boldface, the emphasis is correctly achieved by temporarily replacing the current typeface. Professional typographic systems (which include most modern computers) would therefore not simply tilt letters to the right to achieve italics (that is instead referred to as slanting) or print them darker for boldface, but instead use entirely different typefaces that achieve the effect. As can be seen in Fig. 1, the "w" letter, for example, looks quite different in italics compared to the regular typeface.
As a result, typefaces therefore have to be supplied at least fourfold (with computer systems, usually as four font files): as regular, italics, bold, and both bold and italics to provide for all combinations. Professional typefaces sometimes offer even more variations for popular fonts, with varying degrees of blackness. Only if such fonts are not available, the effect of italics or boldface should be imitated by tilting or blacking the original font.
In Germany, a different means of emphasis was used earlier. To achieve a variance in blackness, instead of making the letters darker, one would increase the spacing between them. This resulted in an effect reverse to boldface: the emphasized text becomes lighter than its environment. This was referred to as sperren in German, which could here be translated as "spacing out". While sperren normally means "to lock (out)", this particular meaning was figurative: with the older method of typesetting with letters of lead, the spacing would be achieved by inserting additional non-printing slices of metal between the types.
The reason for this particular German typographic convention must be seen in the traditional use of Fraktur typefaces, for which boldface was not feasible, since the letters were very dark in the first place. With its demise after Nazi Germany (see Fraktur for details) went the use of spacing as a means of emphasis not long after.