The butter knife.
In Australia, we do also have a butter knife. It is a regular metal knife without the serrated edge. Some people own them, some do not. The average person probably doesn’t even know the difference between a butter knife and the other knives in their drawer. No one would care if you used the butter knife or a regular knife to spread butter, or any other knife for that matter.
In Sweden, they have a butter knife – a specific knife used only for butter. It is plastic or wooden and is the only appropriate tool to be used to spread butter. Do not use a regular knife to spread butter!
See below example of a plastic butter knife. I also have a beautiful wooden butter knife back in Australia with a carving of a Dala horse (a gift from a friend who has family in the Dalarna province).
Butter – smör
Knife – kniv
Horse – häst