Reed organs work by moving air over a metal reed which vibrates at a speed that is governed by its length and mass.  The longer reeds produce lower frequency since their cycle of vibration is longer.  A thicker reed also vibrates slower, producing a lower frequency.  To tune a reed to a higher frequency (raise the pitch), you can lightly file some metal off the movable end of the reed, decreasing its mass and allowing it to move faster.  In order to lower the frequency, you can file material from the fixed side of the reed, causing the remaining mass on the moving end to move slower. 

I use a 80-320 grit sandpaper and gradually remove material from either side of the reed depending on which direction I am trying to move it.  I found that I could move the pitch by as little as one or two cents at a time, allowing me to gradually tune the reeds to this JI tuning with incredible precision.  Stringed instruments are much easier to tune, but they also go out of tune much easier.  The mass and shape of a piece of metal aren't going to change nearly as much over time or with the whether as a stretched hair-thin string attached to a quasi-living piece of wood.  All told, this is not only the most in-tune of any of my acoustic instruments, it's also going to stay in tune for the foreseeable future.  Thus, it is very useful as a reference for tuning other instruments and voices.