We focused on glass screen protectors because protective films are less scratch-resistant, take away from the look and feel of the iPhone’s display, and have been largely supplanted by affordable glass protectors.

We skipped testing glass from well-known brands, including Belkin, BodyGuardz, and Zagg (Zagg also owns the InvisibleShield line). These protectors are widely available, but they’re almost always expensive, and in our tests with glass protectors for previous iPhones, we found their glass to be of the same quality at best, and sometimes worse. More expensive protectors offer no benefits over the cheaper ones we recommend.

For each screen-protector model we tested, we followed the instructions in the box to install it on the iPhone X. We evaluated how clear the instructions were and how helpful any installation tools proved to be. Once the screen protector was in place, we visually inspected it to look for any obvious degradation of screen quality or brightness, and we ran our fingers along the perimeter, checking for obnoxiously sharp edges. We used the phone in a normal fashion to confirm that the protector didn’t interfere with use of the touchscreen. During this use, we also observed how readily fingerprints showed on the protector, and then we wiped off the screen (using a cotton T-shirt) to see how easily we could remove those fingerprints.

We then broke out our Mohs hardness test kit. The Mohs test evaluates the hardness of a mineral based on if it can be scratched by a set of reference minerals, each rated on a scale from 1 (talc, the softest) to 10 (diamond, the hardest). This isn’t the test that screen-protector manufacturers use or advertise; they instead rely on something called the ASTM Film Hardness by Pencil Test, which is a test designed for coated materials. We don’t think the pencil test is the best way to evaluate screen hardness because even the hardest pencil in this test (9H) is softer than tempered glass, so any decent glass screen protector will get a 9H rating, and the pencil test thus won’t reveal differences in scratch resistance between models. More important, many of the things that might scratch a screen or a screen protector are harder than a 9H pencil; the Mohs kit includes materials that can scratch tempered glass.

To confirm that the Mohs test would be valid, we asked Ivar Reimanis, PhD, Herman F. Coors Distinguished Professor of Ceramic Engineering and professor of metallurgical and materials engineering at the Colorado School of Mines. Although he did temper his response by saying that the Mohs test wasn’t a perfect analogue to the pencil test, he concluded that “the ranking would be similar, since you are just evaluating the resistance to scratching with two different probes.”

In the Mohs test, we started with the number 1 stylus, holding it at about a 70-degree angle and applying a moderate amount of force as we dragged it down the glass multiple times in varied directions. (We couldn’t guarantee precisely consistent pressure across trials, but we did our best to use a similar amount of force each time. Unless two protectors were very close in their hardness, the variance in pressure was unlikely to hide the difference, as we saw little ambiguity in the tests—when something scratched, it was very evident.) For each screen protector, we repeated this test with increasing reference picks until a pick scratched the glass. We then recorded the number of the hardest stylus that didn’t scratch the glass, along with the number of the stylus that did scratch it. Because we didn’t have styluses with hardness ratings between integers, in this guide we note for each protector the range of hardness (for example, “between 6 and 7”) that includes the actual scratch resistance of that model.

We didn’t do any sort of drop tests with the screen protectors, because we don’t think such tests are particularly relevant. Tests in which a heavy object, such as a ball bearing, is dropped directly onto the screen don’t represent real-world scenarios—not to mention that an impact that breaks a screen protector may not have broken the phone’s unprotected screen. (We couldn’t easily measure the strength and hardness of that screen, and proprietary data on screen glass isn’t available.) In addition, it’s impossible to re-create every kind of impact a phone may suffer, so the results of a particular type of drop test can’t be generalized to cover all scenarios.