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When you lock a layer, it prevents any changes from being made to the layer. This can be helpful to ensure that nothing on that layer is accidentally altered. Locking a layer can also be used to indicate completion.
You can also lock layer transparency. Locking transparency has a different effect than locking a layer.
Pay attention to any warnings that appear. They will explain what is locked and needs to be unlocked to perform certain actions.
Locking layers in Sketchbook Pro for desktop
In the Layer Editor, tap the layer's marking menu 
then gesture down and to the left to 
Lock.
Simply repeat the process to 
Unlock the layer. This icon is a toggle, switching back and forth between locked and unlocked states.
Locking layers in Sketchbook for mobile devices
In the Layer Editor, tap a selected layer or double-tap an unselected layer to open the Layer Menu.
Tap Lock Layer. Repeat the process to Unlock the layer. This icon is a toggle, switching back and forth between locked and unlocked states.
