hardnessrule
nounUK /ˈhɑːdnəs ruːl/
US /ˈhɑːrdnəs ruːl/
A hardness rule is a rule that says how hard something is.
The teacher used a hardness rule to check which rock was the strongest.
OTHER MEANINGS
2. A rule that says how hard a material must be to be used.
The factory follows a hardness rule for making metal parts.
3. A simple way to compare how hard two things are.
We made a hardness rule to see which candy was harder.
4. A rule about how hard you can press or hit something.
The hardness rule says you cannot hit the wall too hard.