Which tool is essential for aligning a window frame accurately?

Using shims and a level is essential for aligning a window frame accurately because this method ensures that the frame is both plumb (vertically straight) and level (horizontally straight). Shims are small pieces of material that can be inserted into gaps to adjust the position of the frame, compensating for any irregularities in the rough opening of the wall. A level is used to verify that the frame is aligned properly, allowing for adjustments to be made with the shims until the window sits perfectly in place. Accurate alignment is crucial for proper operation and sealing of the window, helping to prevent issues such as air leaks or water intrusion.

Other tools like a hammer, saw, or chisel do not serve the primary purpose of achieving precise alignment. The hammer is typically used for driving in nails or adjusting items that are already in place, while a saw is meant for cutting materials to size. A chisel is mainly used for fine adjustments or detail work but does not provide the necessary level of alignment for a window frame.