Cover labeling of the modifier key and the individual keys: show the correct mode for virtual keypads, never caps labels when in lower-case, put the shifted label above it and gray if needed, as on a hardware keypad, etc. Make sure to keep consistent: avoid having one lock key and the rest use press-twice, but if you do then make sure there's a reason like access to the fn number pad is needed sometimes; never switch modes between virtual and hardware keyboards, or when you switch between keypad and keyboard, etc.
Problem
Provide access to additional and alternative controls without taking up more hardware or screen space through the use of mode switching.
Solution
Modes
etc.
The same principles may also be applied to switching modes in any interface. However, Tabs are the most common implementation outside of character entry. The types of modifiers discussed here only truly become a pattern when used with Keyboards & Keypads, Pen Input, and related functions such as the Dialer.
Variations
Toggles: Dedicated key, with sub-modes Press and hold, press-twice, or dedicated secondary lock keys.
Not-toggles (need a name): Like triple taps mode selectors
Both work on virtual and hardware keyboards and keypads, and can be used as the mode selector for pen input methods as well.
Also discuss tabs for major modes, like handwriting input: often have dedicated keys but they are the same size and adjacent, so they function as tabs, which see.
Interaction Details
Always sticky. In the sense of accessibility keypads. You never have to hold a key down, to get a modifier. So simple shift moves from being a modal modifier (on a desktop) to a single input modifier, and lock is needed for multiple key input.
Multiple lock modes... In variations?
Presentation Details
Indicate locked. For virtual, change the symbol AND change the keycaps to reflect it... For hardware, generally on-screen, see the mode indicator pattern
Antipatterns
Do not implement a "Shift Lock." Older typewriters had no concept of a caps lock, but allowed the shift key to be locked. This also would type the alternative special characters for symbol and number keys. It is not the expected behavior today, and should be avoided.