Oracle 11.2.0.4

Test case:

create table t1 ( a number, x number, y number );
create table t2 ( b number, x number );
create table t3 ( c number, y number, z number );
create table t4 ( d number, z number );

This query works:

SELECT *
FROM t1
JOIN t2 USING (x)
JOIN t3 USING (y)
JOIN t4 USING (z)

This one does not:

SELECT *
FROM t1
JOIN t2 USING (x)
JOIN t4 USING (z)
JOIN t3 USING (y)

I was unable to find an explanation of the reason in Oracle manuals, but I think there is one somewhere.
What am I missing?

Have a nice day and thank you for your time

Mauro

SQL> SELECT *
  2  FROM t2
  3  JOIN t4 USING (z);
JOIN t4 USING (z)
               *
ERROR at line 3:
ORA-00904: "T2"."Z": invalid identifier

SQL> SELECT *
  2  FROM t1
  3  JOIN t2 USING (x)
  4  JOIN t4 USING (z)
  5  JOIN t3 USING (y);
JOIN t4 USING (z)
               *
ERROR at line 4:
ORA-00904: "from$_subquery$_003"."Z": invalid identifier