I've done these with both sides the same width. This makes it easy because you can use a HST for the 'Y' seam corner. I've never seen them as two different widths, but any width is a personal preference.

I've always seen a quilt like this done with the same fabrics for all the window panes. But I don't think they need to be shades of the same color. Here's one I did from a weird fabric in our charity quilt stuff.

With that said, I think by using shades of the same color you gain a better 'window' perspective...

Nancii I have a book here by taking a quick look as long as a light and dark are used its fine, where you put them has something to do with depth wether the kitten look far back or up front. The sill looks to be the same size as the side strip and seems to be 2" or 3" they seem to sew one piece on stopping 1/4" from the bottom of the middle sq. sew the other side on and then folding it over and and sew from the place you stop in a diagonal line.
Now that really is clear as mud isn,t it.
Gillian

Here's a tutorial with photos (I love photos) showing the technique that Gillian mentions.

CMC

Here is a link that might be useful: Sewing Attic Window Blocks Tutorial

I agree that the window elements should be two shades of the same color, except if you are doing a really scrappy quilt.

The width of the sill and frame should be in preportion to the square window area. Too thin and it looks insubstantial; too wide and it overwhelms the "window" or the kitty as in your case.

Teresa

window elements should be two shades of the same color Nanci
But when I did these I did a white for my hst so it looked like snow with my focus fabric.You could play around with the window to see what you like...........no quilt police

Here is a easy pattern no 'Y' seam corners

Here is a link that might be useful: Link

I saw an attic window quilt of a Christmas tree made to look as if we were outside looking in. Each window was part of the tree and all together there was one tree. I don't have a picture unfortunately.