Inside Mount: Inside Mount refers to a window shade that is mounted on the inside of your window frame. This type of mount is by far the most common and is highly recommended. An Outside Mount is generally only used if there are obstructions inside of your window-frame (e.g. an alarm sensor) or if your window frame is not deep enough to hold your shade. You will need at least 2.5 inches to install your shades with an Inside Mount. When measuring, it is important to use a steel measuring tape and round down to the nearest 1/4 of an inch increment. STEP 1: Measure the width of your window; rounding down to the nearest 1/4 inch increment. This will be the width you will order. Please do not make any changes to this measurement. If your measurement is not an exact inch measurement (for example, 35.25 inches), please order the next size up (36 inches in this case) and send us an email with your exact measurement. If we do not receive an email within 2 hours the order placement, we will produce the shade to the size ordered. Outside Mount: Outside mount shades are mounted outside the window opening on either the wood trim frame or adjacent flat surface area. You will need at least 3 inches of flat space is around your window to install your shades. When you specify an outside mount, we will not take any size changes – unless you have contacted us to change the width of the shade within 1 inch of the measured size. STEP 1: If you have a frame or trim around your window, simply measure to the far edges of your trim from left to right. If your window opening has no trim around it, measure a distance from point A to point B that includes any extra width you may desire for light gap coverage and privacy. We recommend adding 2-3 inches to each side. This will be the width that you will order. Please contact us with any questions or concerns. ~Product SKU # WS-1901-0-1-0-0-58x54-115 ~