Patent letter of assurance
The concept of patents is based on a large degree of give and take. The inventor, in return for the reward of exclusive use of the patented invention, is expected to share the knowledge, sooner or later, with the rest of society. It is a permitted monopoly, but it is certainly not supposed to be anti-social. And communication with an inventor whose idea you are concerned you might be about to infringe could save everyone time and trouble. A patent letter of assurance could set your mind at rest and set you on the path to your own patent.
That's not the only kind of patent letter you might have to send or receive, though. Under US law, in the case of a suspected patent infringement you are required to obtain a formal legal opinion, then send the potential offender a 'cease and desist' notice. This patent letter gives both parties the chance to resolve the dispute without going to court, which can be costly and risky on both sides. On receipt of your patent letter the infringer might agree simply to stop if you let the damages drop, or you might be able to insist on compensation. Whether it is worth going to court from the patent owner's point of view depends on how well the claims have been drawn up - if the alleged infringer can argue that the patent prohibitions do not cover their product, they may walk away leaving you to pick up the tab.
At the time of writing patents have to be published in the UK , although in the US you can request exemption from this. In theory, only inventions which really deserve it will be granted a patent, although it is contested how much this is the case in practice for overworked patent offices. A full patent search to determine the novelty of your application does not just cover other patents, but what is called 'prior art'. This is any relevant technology described publicly more than a year before the patent application. For prior art to be such, it has to be in the same field as your invention and it has to have been made public, but it can take a bit of lateral thinking - even an old drawing might count. A detailed and reliable search, then, not only requires time and thoroughness but an expertise in patent law - for this reason it may be worth hiring professional searcher or patent agent to do it, although this could cost several hundred dollars/pounds.