The human mind constantly searches for patterns in everything that it sees everyday, striving to find the unified picture as a whole.
If the mind cannot find pattern in what it is seeking, it has a high tendency to look elsewhere.
When doing web design, it is important to design the website in such a way that makes it easier, not harder for the mind to see the pattern.
It must be clear to a visitor he is still at the same site.
If objects with similar functions are grouped together, its function will be easily understood and remembered.
When designing an interface, it is important that all similar tools used for navigation and accessing information should stay consistent.
The webpage font, style, colour, layout, mouse over effects should also stay consistent.
When the user clicks a link, he must recognize the connecting page is similar to the previous one, failing to do so will make the user confused and will prompt him to visit other websites.
If there are sound or animation used in navigation, the sound and animated effect should stay the same in all pages.
For example, if an animation is turned on by clicking a flash button, it should be done the same way in the rest of the webpages.
One method of ensuring consistency is to use Cascading Style Sheets (CSS).
By defining standard style properties in another document, it is possible to achieve the same look and feel to all your webpages that use the same style sheet.
A style sheet, for example, can be used to make the text on every page 12 pt Verdana, all headings (h1) orange..
etc.
If the style of the site needs to be changed for any reason, only one document needs to be updated.
Without style sheets, doing manual adjustments on every page is tedious and often result in errors and inconsistency.
Another way to promote site consistency is to use templates.
A website template is a document that defines the structure of muliple pages in your site.
For example, if the header and footer are standard across all pages in your site, they can be attached to the body of your document via an automated process, saving you time and typing errors.
Many web editing softwares such as Dreamweaver offer templating function.
Server-side technologies can also do the same job easily.
Consistency is very important in website design because it makes the reader do less thinking.
Time is money.
Your reader wants to find the information he needs quickly.
Inconsistency design only slows down their surfing speed and make them lose interest in your site easily.