Fixnum values (numbers smaller than native one machine word minus 1 bit) are stored in an object pointer. When you do object.id it returns the address stored in that pointer.

For Ruby to know that the value in a pointer is a Fixnum (and not a pointer to an address), it will tag the first bit w/ 1 and shift the integer value by one bit. So storing the value 7 will be done like this:

  (7 << 1) + 1 # => 14

This is why 7.id == 14.

You can see where this is implemented right here: https://github.com/ruby/ruby/blob/trunk/include/ruby/ruby.h#...

For true, false and nil they are also stored right in the object pointer using special values defined here: https://github.com/ruby/ruby/blob/trunk/include/ruby/ruby.h#.... You'll notice that those 3 values in binary format all end with 0b...10. First bit to 1 means Fixnum, second bit to 1 means special value: true, false or nil.