Single quotes vs. double quotes when setting a pwd and other cmds

Hello,

why aren't double quotes accepted below?

db1=# alter user user1 password "pass1";
ERROR:  syntax error at or near ""pass1""
LINE 1: alter user user1 password "pass1";
                                 ^
db1=# alter user user1 password 'pass1';
ALTER ROLE

Is there a thumb rule to know when to use which quotes?
I'm often confused by them when using psql.

Thank you
Alex

PS: Using postgresql-server-8.3.6 @ OpenBSD 4.5