Transaction lock granting order

Can someone point me to documentation on (or confirm) this detail on
Postgres locking?

- Transaction X starts and acquires a lock on a table T
- Transaction Y starts and attempts to acquire a conflicting lock on T - it
is now blocked
- Transaction Z starts and also attempts to acquire a conflicting lock on T
- it is now blocked

Is txn Y guaranteed to be the first txn to proceed once X finishes? Is
there some "lock queue order" that is respected? Is there any chance Z can
proceed before Y, say if it acquires a less aggressive lock?

Put differently: when txn Y starts, it has to wait. Is this wait time at
most the length of the longest txn older than Y?

Thanks,
Josh