Patently Apple:

Today the US Patent & Trademark Office published a patent application from Apple that relates to the field of wearable electronic devices including a wrist biometric sensor. While options for a biometric authentication system for Apple Watch could include Face ID or Touch ID in the future, Apple’s latest patent application on this subject matter actually points to an all new form of biometric authentication using a wrist biometric sensor located on the inside portion of a watch band that reads skin texture patterns.

Imagine if your Apple Watch “knew” your wrist, used its skin texture patterns to authenticate. No iPhone required, no passcode either. My guess is, there’d be some training required the first time you put on your new Apple Watch, in the same way as you trained your face for Face ID or fingers for Touch ID.