Consistency is perhaps the most important concept of design in general, and writing (which would probably be called "text design" if it were a less ancient profession) is no exception.
Inconsistent use of terminology can actually confuse readers; some other consistency issues are just ugly.
Terminology
If you use any technical terminology in your writing, it is vitally important to be consistent. For example, in a project I was involved in at work, we were at one point using the word "channel" to mean three different things! (We've now got rid of the things that I called channels, come up with another name for the things that somebody else was calling channels, and told the third person to shut up about the things he called channels.)
One frequent case of this problem in Web sites is when referring to the site itself. People often have problems with "page" and "site", or "page" and "section", etc. Whichever definitions or names you use for these things, stick with them throughout the site.
Internet style
Various words can be spelt in different ways. Your page will look more professional if you're consistent.
Internet-related words seem particularly susceptible to this problem. My personal opinions (not universal facts!) about the "correct" alternatives for a few of these are:
- email (e-mail is acceptable, E-mail and Email are not)
- Web (web is acceptable)
- Internet or Net (net and internet are acceptable)
- intranet (Intranet is not acceptable)
- Web site (Web site and web site are acceptable, Web site is not; internet site is not if you're talking about a Web site)
Consistent phrase types
Headings and list bullet points should be consistent with each other. For example, this list is particularly poor:
- an item
- Another item
- A third item.
- inconsistently.
- item 4
- blue
- What about all the other items?
All those items are inconsistent with each other. Some are capitalised; some have a full-stop; some include 'a' or an equivalent; some do not fit in the same place within the sentence ("inconsistently", "blue", and the full sentence are different from the "item"s and each other). This is bad style and should be avoided.
(On this site, you might notice that I haven't been able to remain consistent with the style of headings. As I mentioned in the introduction, this isn't an example of a perfect web site.)
