Short answer: 9H on a screen protector is a pencil hardness figure, and the pencil scale stops at 9H. It is the top of a small scale, not a high score on a large one — and it tells you about scratching, not about breaking.
What the pencil test is
It is a coatings test. You take graded pencil leads, from soft through to hard, push each one across the surface at a set angle and load, and record the hardest lead that does not leave a mark. The grades run 6B at the soft end up through HB and on to 9H at the hard end. 9H is the last pencil in the box.
So a 9H result means the hardest lead in a pencil set did not scratch it. It does not mean the surface sits at 9 on some open-ended scale, and there is no 10H pencil to compare against. A protector advertised at 10H is claiming a grade that the test it is quoting does not have.
Hardness is not toughness
These are different properties and they often move in opposite directions. Hardness is resistance to being scratched. Toughness is resistance to being broken. Glass is hard and not especially tough — that is why a sheet survives a year in a pocket with keys and then shatters when it meets a kerb.
A protector that cannot be scratched by a pencil can still break when you drop the phone. Both things are true at once and neither is a defect.
What actually scratches a screen
Not keys and not coins — both are softer than glass. The culprit is sand and grit, which is largely quartz, and quartz is hard enough to scratch glass. A grain in a pocket, in a bag, or on a table under the phone does more damage than a keyring ever will.
Which is the useful thing about a protector, and it has nothing to do with the number: the grit scratches a sheet you can replace for the price of a coffee instead of a display you cannot.
What to look at instead
Fit, because a sheet that is a millimetre out lifts and takes dust underneath. Adhesive across the full face rather than a frame, because that is what keeps an in-display reader working. Edge treatment, because a sharp edge catches on a case. Thickness, stated honestly. And the replacement policy, because the sheet is a consumable and it is going to need replacing.
We state 9H because it is the figure the industry uses and leaving it off invites the question. We would rather you bought on the cut.
Questions people ask
What does 9H mean on tempered glass?
It is a pencil hardness grade: the hardest lead in a standard pencil set did not scratch the surface. The pencil scale stops at 9H, so it is the top of a small scale — and it is about scratching, not breaking.
Is 10H or 11H tempered glass better than 9H?
There is no such grade. The pencil test that 9H comes from ends at 9H, so a listing claiming 10H or more is quoting a number the test does not have.
Does 9H mean the glass will not break?
No. Hardness is resistance to scratching; toughness is resistance to breaking, and glass is hard but not especially tough. A 9H sheet can survive a year in a pocket with keys and still crack when the phone meets a kerb.
What actually scratches a phone screen?
Sand and grit, which are largely quartz and hard enough to scratch glass. Keys and coins are softer than glass and are rarely the cause.
What do 11D and 21D mean on tempered glass?
Nothing measurable. They are marketing names for edge and coverage styles with no standard behind them — unlike 9H, no test produces them. Check that the sheet is cut for your exact model and suits your screen’s edge instead.