Audio interface are mainly divided in to four interfaces: 3.5mm, 6.5mm, RCA, XLR.
6.5mm(also known as 6.35mm interface) is an old and long-lasting interface which is first be invented to use on telephone switchboard in 1877. In 1902, Henry P. Clausen officially applied a patent for the 6.35mm interface and its application gradually spread into the field of various audio equipment like power amplifier, electronic guitar, etc. 3.5mm came in 1980s and it’s the direct descendant of 6.5mm interface. Due to the smaller size, 3.5mm is much suitable for the use of portable audio devices like MP3, mobile phone, etc.
The name of RCA derives from the name of its inventor: Radio Corporation of America. In 1937, it was first released to be the internal connector of radio-phonograph floor consoles and gain popularity on the devices like TV.
XLR connector is a type of electrical connector primarily found on professional audio, video, and stage lighting equipment.