Web-based credit company NextCard sued credit giant Providian for copyright infringement in a case that could set a precedent for how the law applies to Internet advertising.

In the suit, filed Wednesday in U.S. district court in San Francisco, NextCard accused Providian of copying a Web banner ad that features a descending thermometer to represent falling interest rates. NextCard said the ad is its most effective, with about 680,000 Web users clicking on it.