Here we have a common PD ruler. PD stands for pupillary distance, which is the measure to the center of each pupil. Which can be done from the center of the nose to the center of the pupil or taken binocularly which is from the center of one pupil to the center of the other pupil. PD rulers come in both plastic and metal like this one here and have nice clear markings on them like you can see in front of you to take any lens or frame measurements needed. The only downside to having a metal PD ruler like this one is if you're not careful when you place it on a patient lens in order to take measurements you do risk the potential of scratching that lens. It’s also important to note that a lot of times you can get PD rulers for free from your lab so make sure you're not using a PD ruler that’s old with the numbers worn off rather than just replacing it.