We use EIA Standard RS-297-A, which describes the use of the three-pin XLR - known as XLR3 - for balanced audio signal level applications, with pin function as follows:
1 Chassis ground (cable shield)
2 Positive polarity terminal for balanced audio circuits (aka "hot")
3 Negative polarity terminal for balanced circuits (aka "cold")
Prior to the introduction of this standard, the wiring of pins 2 and 3 varied. The pin 2 "hot" and pin 3 "cold" convention was typically used by European and Japanese equipment manufacturers, but American companies used pin 3 "hot" and pin 2 "cold". This caused problems when interconnecting equipment with unbalanced connections. The pin 3 "hot" convention is now obsolete but is still found on vintage equipment. Pin 1 has always been ground and/or shield if the cable is shielded, and many connectors connect it internally to the connector shell or case.