

A passcode is required to set up Touch ID, and Touch ID is automatically disabled after 48-hours until a passcode is input by an iPhone or iPad's owner. In the United States, passcodes are especially important because the law suggests that while law enforcement officers can require you to provide a fingerprint to unlock a device, the same is not true of a passcode.
For a long time, passcodes were four-digit numeric codes by default, but with iOS 9, Apple began using a six-digit passcode as the default option. Six-digit passcodes offer 1 million possible combinations instead of 10,000, making a passcode harder to crack.
Apple doesn't advertise it, but the iOS operating system offers an option to make your passcode even more secure through the use of an alphanumeric passcodes or custom length numeric passcodes. Alphanumeric passcodes contain letters and numbers. Both alphanumeric and custom numeric passcodes can be much longer than four or six digits.
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Article Link: How to Create a More Secure Passcode on Your iPhone or iPad