No tape measure required. For the right size, find the window code reference located at the top of the open VELUX roof window. Compatible with VELUX roof windows with reference codes: GGL F06, GGU F06, GHL F06, GHU F06, GPU F06, GXL F06. An inexpensive option from VELUX, ideal for replacing old and worn blinds. Manually operated with stepless positioning for an almost complete blackout effect. Easy installation – Tailored for VELUX roof windows for a perfect fit. Install in minutes with the pre-fitted mounting system. If mounting brackets aren't pre-fitted, you'll need to purchase the Z0Z 230 separately.