What is the effect of a proper foreign priority claim on the effective filing date?

A proper foreign priority claim can give a U.S. patent application the benefit of the earlier foreign filing date as its effective filing date for prior art purposes. This can be crucial for establishing novelty and non-obviousness of the invention.

35 U.S.C. 119(a) states that a U.S. application properly claiming foreign priority “shall have the same effect as the same application would have if filed in this country on the date on which the application for patent for the same invention was first filed in such foreign country.”

This means that intervening prior art published after the foreign filing date but before the U.S. filing date may not be used against the application if the priority claim is valid. However, it’s important to note that the actual U.S. filing date is still used for other purposes, such as calculating patent term.