The effect of co-citation is often overlooked by search engine optimizers. The other web sites to which your link partners link can influence your search engine rankings.
Here's an example: web sites 1, 2, 3 and 4 all link to the web sites A, B, C and D. Although A, B, C and D don't link to each other, search engines think that A, B, C and D are related to each other because the same web sites link to them:
If A, B, C and D are all linked from 1, 2, 3 and 4 they might be related to one another, even though they don’t directly link to each other. If A, B, C and D are all linked by many other web sites, they have a strong relationship. The more web sites they are linked by, the stronger the relationship.
If your web site is web site A, you should make sure that web sites B, C and D are related to your site.
Suppose you have a web site selling shoes. If you are listed on a web site that links to gambling sites, viagra sites and your shoe store then search engines might think that your web site is related to gambling and viagra.
The search engines look at the link pages and check to which other web sites the pages link. If the other pages are gambling and viagra sites then search engines think that your web site is also related to gambling and viagra.
That means that it might be difficult to get high rankings for search terms that are about shoes.
It's important that the other links on the web page that links to you are related to your site. If you're listed in the "Shoes" category of a link directory, all web sites in the same category are usually also about shoes. When search engines look at this page and check the links to other sites, they will think that your web site is related to shoes.
That means that it will be much easier to get high rankings for search terms that are about shoes.
When you exchange links with other web sites, make sure that the link to your site is on a page that lists related links. It's not necessary that the other web site is completely about your topic. It's okay if the other web site has a single category page that deals with the topic of your web site. The link to your web site should be listed on that page then.
The more pages of the other site are about your topic, the better. Make sure that the link to your site is in a good neighborhood and that the other links are related to yours.