Interior window trim is not only a great option for giving old windows a breath of fresh air, but also for adding functionality, keeping out rain and making windows more energy efficient. Read on to learn more about proper window trim care and installation. Anatomy of Interior Window Interior window trim includes the head casing, side jambs, sills and the apron (the piece right beneath the sill). Each element plays a unique role in structurally supporting the window. Installing Interior Trim Installing interior window trim can be a fun and satisfying project to undertake. When you’re ready to begin the installation process, take note of the following tips: