The electric toothbrush and its charger we've had for as long as I can remember have a slightly trapezoidal (actually a circle with two sides cut off at an angle really) peg and slot thing so you can't put it on backwards. We've had it for years. The thing's been positioned so you have to put the thing in facing away for a long while now. You can feel it doesn't fit. And yet someone's forced the thing on so hard I had to put some effort into separating them.