Context: The basic MP3 decoding and encoding technology is patent-free in the European Union, all patents having expired there. In the United States, the technology will be substantially patent-free on 31 December 2017 (see below). The majority of MP3 patents expired in the US between 2007 and 2015.

Question: Where is the basic MP3 decoding and technology patent free?
Answer: European Union

Question: What happened to the patents that caused them to be patent-free?
Answer: expired

Question: When will the technology be patent - free in the United States?
Answer: 31 December 2017

Question: Between which years did most of the MP3 patents in the US expire?
Answer: 2007 and 2015