Context: In September 1998, the Fraunhofer Institute sent a letter to several developers of MP3 software stating that a license was required to "distribute and/or sell decoders and/or encoders". The letter claimed that unlicensed products "infringe the patent rights of Fraunhofer and Thomson. To make, sell and/or distribute products using the [MPEG Layer-3] standard and thus our patents, you need to obtain a license under these patents from us."

Question: When did the Fraunhofer institute send out a letter?
Answer: September 1998

Question: What did the letter claim would be required to distribute and/or sell decoders and/or encoders?
Answer: a license

Question: What does selling unlicensed products mean that the seller is doing?
Answer: infringe the patent rights of Fraunhofer and Thomson

Question: What can products not use if they want to avoid infringing on patent rights?
Answer: the [MPEG Layer-3] standard