Difference between nomount,mount and open database in Oracle

Nomount – The database instance has been started (processes and memory structures have been allocated, but control file is not yet accessed).

Mount – Instance has accessed the control file, but has not yet validated its entry or accessed the datafiles.

Open – Instance has validated the entries in the control files and is accessing the datafiles – it is now open for business.

The startup process is Nomount -> Mount -> Open.