Grid windows with inside grids are available for purchase. Because the grid's dividing parts are "sandwiched" between the glass panes, these grid systems are a little cleaner. This grid window is more attractive, but it is also quite sturdy. The glass panels can be removed or replaced easily if needed. Windows with frames but no glass are used for very cheap single-pane windows and come in a wide variety of designs. They're easy to make and inexpensive to buy. The main advantage of these windows is that they're lightweight and easy to move around the house. The downside is that if something breaks inside the frame, you'll need to replace the whole thing instead of just the glass. Windows with steel frames contain several small holes per inch across the entire surface of the frame. These are called "bull's-eye" holes and provide air circulation on hot days. They're not necessary, but many manufacturers include them as a bonus feature. Wooden windows are the most common type of window used by homeowners. They come in a wide variety of shapes and sizes and are usually installed using wooden screws or nails. Some varieties of wooden windows have metal reinforcements within their internal wood structure to increase their strength. Other types of wooden windows are simply reinforced with bracing material outside of the frame. Still others are filled with insulation! There are windows out there designed specifically for boats and other vehicles.