Can rsync resume after being interrupted?

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http://unix.stackexchange.com – I used rsync to copy a large number of files, but my OS (Ubuntu) restarted unexpectedly. After reboot, I ran rsync again, but from the output on the terminal, I found that rsync still copied those already copied before. But I heard that rsync is able to find differences between source and destination, and therefore to just copy the differences. So I wonder in my case if rsync can resume what was left last time? Thanks! (HowTos)