If you want to use chmod to change the permission of a file or directory. You just need to remember that:

• 0: no permission
• 1: execute
• 2: write
• 4: read

With this you can add up the permission you want to change. For example:

• 7 means 1 + 2 + 4 which is all the permission
• 6 means 2 + 4 which is only write and read

And now you don’t have to remember all the number again.