• X-Y stereo: two directional microphones usually with a cardioid pickup pattern, placed with the microphone elements (front of the microphone) almost touching and at an angle of 90 to 135 degrees to each other.  There are brackets, called stereo microphone bars,  available which attach to a microphone stand and hold the microphones in the proper orientation for this and several of the other common orchestral techniques.
  
  X-Y also depends completely on directionality method one
  
  
• ORTF stereo: (Named after the French broadcasters who developed the technique.) Two cardioid microphones oriented at an angle of 110 degrees to each other, with the elements spaced 17 cm (about 7 inches) apart horizontally.  The microphone on the left should be facing to the left of center, the mic on the right should be facing to the right.  This means that with long microphones, the rears of the two microphones may well have to cross each other.  Using a stereo microphone bar with an ORTF setting will help assure correct spacing and orientation of the microphones.
  
  ORTF depends on both method one and method two.  The spacing between the microphones affects method one, the angle affects method two. 
  
  
• M-S or Mid-Side stereo:  A bi-directional (figure-8) pattern microphone facing sideways to the musicians, which records the stereo difference signal between the left and right, and a directional (often cardioid) mic aimed at the musicians which records a monophonic signal of the performance.  The use of M-S recording requires a matrix to make the left and right signals.  The matrix may be a piece of hardware, or often now a software "plug-in" for an editing program.
  
  M-S directionality is method one.
  
  

There are variations of the above techniques and any number of other stereo miking techniques, each of which has advantages and disadvantages.