What is a Casement Window. The Casement is the part of the window that opens. Casement windows are a type of window that is attached to the window frame which opens and has hinges attach at the top, bottom or to the side. Casement windows are probably the most popular type of window in the UK and have been in production for a long time. The reason casement windows are one of the most popular types of windows is because they are very reliable, durable and energy efficient. They also benefit from being very customisable and will suit of variety of styles and budgets.