The rc0.d,rc1.d,... directories in /etc

I'm running Ubuntu where I have the directories /etc/rc0.d, /etc/rc1.d, /etc/rc2.d, ..., /etc/rc6.d. Example files from my machine: directory example symlinks in the dir ------------------------------------------ /etc/rc1.d: K76dovecot, K77ntp /etc/rc2.d: S23ntp, S24dovecot /etc/rc3.d: S23ntp, S24dovecot /etc/rc4.d: S23ntp, S24dovecot /etc/rc5.d: S23ntp, S24dovecot Questions: 1. What's the purpose of the multiple "rc" directories? 2. Why did Ubuntu install duplicates of dovecot and ntp into all the directories except rc0.d and rc6.d? 3. If they are specified multiple times like above, are they actually executed multiple times? 4. Can you tell from the above in what order dovecot and ntp will execute at startup? 5. What is the proper way to tell Ubuntu to always execute ntp before dovecot at startup?