Some upside-down designed ATX cases are also accessed by removing the right-side panel door. Alternatively, the case may have a single large U-shaped cover that saddles the chassis. This was ubiquitous in AT tower cases. To prevent the buildup of performance-degrading dust within the case, many models feature dust filters in front of the air intake fans. While the exact method of accessing the front filters depends on the case model, it usually requires the removal of the entire front panel. Removal of the front panel itself may or may not require the removal of one or both of the side panels. If there is any doubt, consult the user's manual if possible. Traditionally, most computer cases required computer case screws to hold components and panels in place (i.e. motherboard, PSU, drives, and expansion cards). From the early 2000s onwards there is a trend towards tool-less cases, in which components are held together with snap-in plastic rails, thumbscrews, and other methods that do not require tools; this facilitates quick assembly and modification of computer hardware and is also cheaper to manufacture. Appearance Throughout the 1990s, most computer cases had simple rectangular shapes, and were often painted beige or white with little attention given to the visual design. Beige box designs are still found on a large number of budget computers assembled from generic components. This class of machines is still known as white box computers. More modern computer cases include a much wider range of variation in shape, color, form factor, and materials, such as brushed aluminium and/or tempered glass which are offered with more expensive cases. Case modding is the artistic styling of computer cases, often to draw attention to the use of advanced or unusual components. Since the early 2000s, some cases have included clear side panels or acrylic windows so that users can look inside while it is operating. Modded cases may also include colored internal lighting, custom paint, or liquid cooling systems. Some hobbyists build custom cases from raw materials like aluminum, steel, styrofoam, acrylic, or wood. Historically, cases used CCFL lighting and eventually single color LEDs as strips or in fans to illuminate their interior; modern cases use RGB LED lighting instead, often incorporated into case fans. To improve airflow while allowing RGB fans to be visible, many cases as of 2020 use metal meshes, without any external bays. Many include a PSU shroud and