The curved lines of an arch or semi-circle window provide a great architectural detail, breaking up the straight lines and corners of most walls and windows. However, these windows present challenges due to their irregular shape for those who want to cover them with a window treatment.
Many window-treatment companies offer various products for arch windows that can be made to match the other blinds in your house. Most will give you a choice of movable or stationary blinds. Movable blinds are good for windows you want to open and close, or windows that have highly variable light. Stationary blinds are good for high-up windows and windows that remain closed. The common solution for arch windows is the honeycomb or cellular blinds. Cellular arch covers are fan shaped to cover the curve of the window. They can be made out of many materials but are most often made of a light-filtering or blackout fabric and come in many colors. Many honeycomb arches are movable and can be open and closed like a fan. One of the more expensive options are fixed wood or PVC slats. You might want to consider that if you already have a slat blinds on your other windows.
If you are shopping on a tight budget, the company Redi Shade makes an affordable, easy to install, pleated arch window treatment called the Redi Arch. The Redi Arch is a temporary window treatment made of paper that comes with a self-adhesive plastic mounter. They look nice and professional and take about 5 minutes to install. If you're not into white paper, Redi Shade also has instructions on their website for how to customize the Redi Arch with fabric or paint to make it match your decor. The Redi Arch is probably your best bet for a professional looking window treatment at low cost.
Depending on your reasons for covering your window, you may also want to consider window treatments that don't involve blinds. If you are looking to protect your furniture from fading and light damage, there are UV blocking films that you can apply to your windows. For privacy, there are many decorative films available that can give your window a variety of looks from frosted to stained glass to crackled glass and other textures in many colors. The company Decorative Films offers samples of their product by request through their website. Window films are fairly affordable and easy to install. All you have to do is cut them to size and stick them on the window. If you are crafty and feeling bold, Gallery Glass is a product that can give your windows the look of stained glass. It takes some finesse to apply properly, but looks amazing when done well.
Factors such as height and location of the window and a desire for privacy or light-blocking will help you decide which method is right for you. As you can see there are many options and many of them are very affordable.