Originally posted by bluemana
My version doesn't have it... Only has option to choose the axis (x,y,z.-x,-y,-z), join vertices or connect vertices (enable disable), but no threshould.
the threshold is not under the mirror tool options - it's under the merge tool options, which is a seperate tool (under "edit mesh").
The merge function on the mirror tool will use the same threshold though.
All versions have it I believe.