What's cuter than a platonic solid? A cat dressed up as a platonic solid for Halloween!

This instructable details the making of a soft quilted tetrahedron costume for the more patient of my two cats.

Step 1: Choose Fabrics and Preprocess Them

The first step in any quilt is to pick out fabrics. You will need at least 4 colors, two for the contrasting triangles, one for the outside border, and one for the backing. I was working with fabric scraps from other quilts, so I used more colors but whatever you've got will work. Be sure to wash, dry, and iron all fabrics at least once before you start working with them. There's nothing worse than finding a finished product ruined in the wash by runny dyes or fabric shrinkage.

You'll also need an attractive color of quilt binding. Binding can be purchased at most fabric stores (I use Joanne's). The binding is a cloth strip that will sandwich the outer hem to create a finished edge. Also make sure you have a color of thread that matches your fabrics.

Step 2: Cut Triangles and Piece Them Together

Using a rolling cutter, measure and cut equilateral triangles from the chosen fabrics (the size will vary depending on the size of your cat). You will need 9 of one color and three of the other. Pin the triangles together one pair at a time, sew, and iron the seams flat immediately. Never skip the pinning or ironing steps! Quilting is all about patience; you will get far far better results if you are methodical.

Measure a strip of your 3rd contrasting fabric for the outside edge; the border shown in the pictures is a 2" strip. When the 4 triangles are sewn together and ironed into a bigger triangle, pin the border on the edge and sew in place. Once one edge is attached, cut the edges flush, iron flat, then repeat for the other edges.

Step 3: Attach Batting and Backing Fabric

Take the backing fabric and lay it flat on your work surface, right side down. Place a layer of quilt batting over the fabric, taking care to smooth out any wrinkles. I used polyester low loft batting in a "child's blanket" size. Anywhere that sells quilting supplies will have a ton of options for batting lofts (how thick it is) and material (cotton, polyester, etc.). A lower loft is better for this project because it will keep the overall product lighter and be easier for the cat to wear.

Place each pieced top right side up on top of the batting, making a batting sandwich, and pin it into place with basting pins (which are like safety pins, but shaped more conveniently for this purpose). Once the batting is pinned on, cut the sides of both the batting and backing fabric flush with the quilt top. It's easiest to use a rolling cutter and straight edge to do the trimming all at once.

Step 4: Attach Binding, Quilt Along the Color Borders

For ease of construction the triangles will all be independently finished and then sewn together later.

Pin the binding strips to sandwich all three edges of each triangle. Fold the edge of the binding under at the corners to hide any unfinished edges. With your sewing machine set to very slow, carefully stitch the binding in place on all 9 edges. As you sew the binding you should aim your stitches at the very edge, no more than a millimeter from where the binding meets the quilt top.

I also chose to quilt another line a quarter inch out from the binding seam. This gave it a nice visual border and took care of holding the binding on at any of the points where I messed up on the initial seam. Specifically, it's easy to have stitches that are accurate on the top, but that miss the binding edge on the underside. The second line of stitches takes care of these cases and reinforces the final product.

While I was at it, I stitched across the seams where the black border met the large triangles, and along the color borders between the inner triangles. This sort of stitching is what gives it that classic quilted look, and also keeps each triangle together flat as it flexes. Try to use one long line for all your quilting stitches; you want as few loose thread ends as possible.

Step 5: Cut Head Opening, Assemble Tetrahedron

Pick one of the finished quilted triangles and cut a smaller upside-down triangle out of it, as an opening for the head. Use a wide sewing machine stitch to finish the cut edges. This was the part of the project I was least happy with the finished look of. If had it to do over again I would have made the head-triangle out of three smaller triangles with finished edges, rather than cutting into it and attempting to finish with sewing machine stitches. However fortunately the border doesn't really ever show while the cat is wearing the costume.

Using a color of thread that matches your binding, sew the edges together. Place triangles back to back (right sides out) and simply stitch along the edge. Though this will create a lip on the outside (showing edge) of the final product, because the triangles are already finished with binding I consider it aesthetically acceptable. Repeat process with the final triangle and the last two edges. After the edges are sewn together you should have a soft pyramid, almost capable of standing on its own.

Step 6: Initial Fitting

At this point I stopped to test how the tetrahedron would look on my cat. My hope had been that the stiffness of the edges would be enough to make the soft tetrahedron stand on its own once it had a cat inside. However as he started walking around it became apparent that a frame was required for it to maintain its shape, especially when you consider how the tail deforms the soft sides. I had thought that the tail could go under the bottom border, but my cat had other ideas.

Step 7: Create Wire Frame

After the initial fitting, I decided the costume needed a frame for full effect. Take three wire coat hangers and bend completely straight using either your bare hands or pliers where necessary. Once you have a long length of straight wire, measure it against the tetrahedron edges and bend it into a triangle, duct taping the edges together. Make sure that the beginning of each wire is not at a corner, but is in the middle of an edge. This will make it much more stable and easier to tape safely in place.

Make three wire triangles the size of the tetrahedron sides and duct tape them together into a pyramid. Make absolutely certain that all sharp edges are firmly duct taped in place so that there are no unpleasantly pokey parts to injure your cat. Forming the pyramid out of three finished triangles has the benefit of creating nice smooth pieces, and also having two wires along each edge will give it extra support because coat hanger wire isn't that strong.

When the frame is ready simply pull the cloth over it and use safety pins or basting stitches to hold it in place. You don't want to permanently attach the frame because you may want to wash the costume in the future. All that you need is something to hold the bottom edge of the cover to the bottom edge of the frame, and safety pins hidden on the inside work perfectly for that.

Step 8: Final Fitting

Once the frame is attached to the quilt the costume it's ready to go! Gently coax your cat into it and admire your work!

At first I was worried that my cat would be unhappy wearing the costume, but as it turns out he really likes it. Like many cats, he's got a strong instinct to sit inside of things so even when I took the costume off of him he got right back in. I would worry about him injuring himself if he wore it while I wasn't directly supervising him, but he definitely seems to appreciate the concept. Also, he appears to be able to walk around freely while wearing the costume, which is adorable!

While your cat is not wearing the tetrahedron it can be placed on its side to make a quilted cat tent-nook-bed thing, which both my cats (even the one that's far too skittish to be costumed) like very much.

Happy Halloween!