Difference Between Oracle Mount, No Mount and Open Stage

NOMOUNT: This stage is only used when first creating a database or when it is necessary to recreate a database's control files. Startup includes the following tasks.
· Read the spfileSID.ora or spfile.ora or initSID.ora.
· Allocate the SGA.
· Startup the background processes.
MOUNT: This stage is used for specific maintenance operations. The database is mounted, but not open. You can use this option if you need to:
· Rename datafiles.
· Enable/disable redo log archiving options.
· Perform full database recovery.
· When a database is mounted it 
o is associated with the instance that was started during NOMOUNT stage.
o locates and opens the control files specified in the parameter file.
o reads the control file to obtain the names/status of datafiles and redo log files, but it does not check to verify the existence of these files.
OPEN: This stage is used for normal database operations. Any valid user can connect to the database. Opening the database includes opening datafiles and redo log files. If any of these files are missing, Oracle will return an error. If errors occurred during the previous database shutdown, the SMON background process will initiate instance recovery