location of user crontab files?

[Identry]

Where are user crontab files stored in the file system?

I used to create user crontab files and then load them with 'crontab
name-of-cronfile', but then I got clever and discovered 'crontab -e'.
The problem is, that doesn't give me the 'backup' of having a text
crontab file.

The system must store the info somewhere so it's available after a
reboot, but where? I want to make sure this info is backed up.

Thanks: John