211 ½ - the most used whistle among retrievers, it is possible to form even lower tones beautifully with it
212 - whistle with longest reach and widely used among pointers
210 ½ - whistle with a higher tone
210 - trill whistle used for work in water, wind, or for the 'lie down' or 'sit' command (stop-whistle)
640,641- two-tone whistles
Acme whistles are a good choice, because their tone is still the same ... unchanging in the case of illness, changing weather or in the case of change of the dog's leader.
If you happen to lose the whistle or if the whistle wears out, you are sure that you will always receive a whistle with the same tone to what your dog is used to.