Dots Per Inch and Pixels Per Inch, DPI and PPI respectively, are units for measuring the tiny bits of color that come together to form a printed image or an image on a screen. The higher the density of those dots or pixels, the higher the resolution. For most, a rudimentary understanding of DPI or PPI amounts to knowing whether or not an image is clear but small enough to print, email, or post to social media. However, for designers, understanding screen density and ratio is essential to delivering delightful user experiences across devices. Designing for phones, tablets, laptops, watches, and hybrids without a proper knowledge of dpi and ratio is a lot like building a house using the wrong measurement—things are going to look very weird very quickly (and good luck getting through the door!).