What is the convention for the star location in reference variables?

Question

I have been learning Objective-C, noting that different books and examples use differing conventions for the location of the star (*) when naming reference variables:

MyType* x;
MyType *y;
MyType*z; // this also works

I see the first two used interchangeably, and sometimes in the same code I've seen differing uses of both. Is one format easier to read, easier to maintain, or less error prone than the other formats, or is it simply a matter of personal preference?