Hi Dirt slinger, Hank pretty much nailed it right on,though there are on occasion there will be those coils that are discovered to be interchgangable "across" different brands. It is pretty rare though and to expensive for a lot of experimenting. It should be noted though that Tesoro is a bit unique, in that, the way that most all of their "machines" are designed/set up, there is a wide range of their coils/machines that are interchangable with other different models of Tesoros. This includes many of the older models and newer models that can use the same coils, This allows a really goodly amount of advantage when hunt sites will call for a smaller coil for trash litered sites , or for larger coils when hitting open fields or woods.Or, simply choose a lighter coil to "winddown" a long day of hunting. Coils can be "tuned" to your Tesoro detector for full potential of that particular coil/ machine combination or be tuned for a "happy medium" of potential with several different coils, and this still allows you plenty of overall machine to" coils" potential.Tesoro design/set up allows for a much greater variety of coils (choices ) for the many different hunt situations you will encounter. It is a big advantage, especially when many of us appreciate that advantage so much that it becomes second nature for us to acquire several different older, or newer Tesoros.That way we always have different combinations of machines and coils for whatever type hunt site happens to come our way. That's cool, and it Works! ! HH, Charlie