In the declaration

    char letter = 'x';

the initialization expression 'x', which for historical reasons is of type int in C, is implicitly converted to the type of the object. `letter` is a `char` because you defined it that way.

If you had written

    int letter = 'x';

that would be perfectly valid, and the conversion would be trivial (int to int).

It's just like:

    double x = 42;

`sizeof 42` might be 4 (sizeof (int)), but `sizeof x` will be the same as `sizeof (double)` (perhaps 8).