US search engine Overture services last week accused the far more popular Google of infringing upon its 'bid-for-placement' product patent.

The patented product basically enables companies to buy their position on a search list by out-bidding rival companies on relevant keywords, such as car or travel. Advertisers also pay the search engine to drive traffic to their sites.

But Google denies the claim. In a statement released on Friday, the popular search engine said, "We have analysed the patent and determined that we do not infringe any valid claim that it contains." Google was unavailable for further comment.

Google introduced a similar system to Overture's in February, although it still continues to reserve most of its site for results sorted by relevance.

"The paid listing and the 'editorial' content results are usually kept separately," said Danny Sullivan, founder and editor of SearchEngineWatch, a site that gives free and paid-for advice on how to get websites into search engine databases. "For the consumer, it's not always clear what is what, but there are divisions."

Which is the case with Google itself, keeping 'paid-for' sites in separate boxes on the right-hand side of the search list.

According to Sullivan, as long as pay-for-performance sites are kept separately, reducing user confusion, the system can be useful for all those involved.

"Some sites may find it unfair that they can't afford to advertise. Unfortunately that's true of any small business versus big business," said Sullivan. "However, by targeting your ads to important but less popular terms, even the small business can still draw good traffic."

But Search Engine Optimisation, or SEO as the black art of getting a website to the top of a search engine rankings list is known, takes place on most search engines according to the UK's Search Engineers.

"Each [search] spider looks for different things — key words, external links, number of hits — so it is important to constantly tailor sites to cater for each engine's preference," said a spokesman.

It seems clear that paying for placement, by whatever means, is common practice throughout most search engines, but the key issue here is that Overture believes Google is using its search technologies.

Overture filed the same suit against search engine earlier this year, but no decision has yet been reached. Both cases are now in the hands if the Californian US District court.