Mid size is one of the oldest stereo recording methods that there is. The article goes into detail about specifics of the method that started in the year 1934 with a patent from Alan Blumlein. a multi channell is created by using different microphones in a specific manner. For a person to benefit greatly from reading the article, it would help for them to be very interested in stereos or to be an audiophile.
Read the full article here:
Mid-Side Explanations (Part 1)