When model binding includes a bool what string value must the form collection contain?

If I am using model binding and one of the variables in the model is a bool what string must the form collection contain to be considered true or false?

So for example if my model has a variable:

bool isHappy;

Now when the model binder reads the form collection and it contains name "isHappy" what will the value have to be? "true", "checked", "1", etc?