Empress Effects ParaEq - Manual (Page 5)

Q Controls
The Q is a measurement of how much the EQ band affects a range
of frequencies.
Tight or Narrow Q ( ): This setting is best for attacking problems.
For example, if you have an acoustic instrument feeding back, a
tight Q will allow you to cut the offending frequency without
affecting the frequencies around it.
Medium Q ( ): This setting is great for general tone shaping. Most
equalizers in instrument amplifiers are medium Q. Try using this
setting to cut frequencies in the 300Hz-400Hz range if your amp is
sounding a little muddy, or boosting in the 1 kHz-5 kHz range if your
guitar is a little dark.
Wide Q ( ): Wide Q settings are best when you want a really
transparent change to the signal. For example, boosting at around
100Hz can add a bit of warmth, and a little boost in the 3k range can
add detail and definition, all while retaining the original tone.