I don't have any hard numbers on this, but I suspect that the reasons for which I use skirts as layers follows this order in terms of frequency:
1. To add length to a too-short dress or skirt
2. To bring in an additional color, texture, or pattern
3. To add opacity or density to a sheer or clingy fabric
4. Because the outfit is sort of boring otherwise
5. To add volume to my lower half for balance and proportion
6. For warmth (obviously I live in a mild climate, but if your winters are more extreme you might use layered skirts for warmth a lot more than I do)
I won't go through all of the scenarios individually, because regardless of the reason behind it, the approach to layering skirts is really the same. All you will need is a skirt, tunic, or dress that is shorter, and another that will bring the combined hemline to the length you desire. If the layers are the right proportions by themselves, great, but alternately you can try hiking one or the other layer up or down, covering the whole mess with a top or cardigan, and belting the layers with an elastic belt or obi to hold everything in place. The photo below, on the far right, is a great example of this approach.
Now keep in mind that not all short/long combinations will work; a very short skirt with a very long one underneath can look weird and out of proportion. In general you probably only need a couple of inches of the lower layer showing even if you're layering to add length to a too-short hemline, but even this isn't really a hard and fast rule. The examples below illustrate the use of a layered skirt to add length (first and second photos) and/or density (third and fourth photos), with varying degrees of the lower layer showing. In general, it depends a lot on how much contrast there is between the layers; the more harmonious they are, the more you can get away with letting the lower layer stick out, because it'll look like it's all one piece. The more the two layers differ in color, texture, or pattern, the more the lower layer will distract from the rest of the outfit if you let too much of it show. The one exception is if you're going for a colorblock look, in which case you'd want to have big swaths of both layers showing.
Let's talk for a moment about using layered skirts to bring in an additional color, texture, or pattern. Let's say you want to wear a solid yellow top with a solid purple skirt, and are looking for a way to make the two pieces relate to one another. You could try either layering a patterned skirt with yellow in it underneath, or layering a solid yellow skirt underneath. In either case you'd have created a bottom half containing both your colors, which would tie them together. Another scenario might be where you have a fairly plain piece, and you want to add some more interest by bringing in a colorful pattern or interesting texture. I use my tulle skirts for this a lot, because they add a soft, 3-dimensional edge to the bottom of a plain hem.
Some important things to keep in mind when layering skirts either under dresses or under other skirts:
- Think of your layered bottom half as a single, integrated piece. I find that mentally, it's much less taxing to figure out what to wear with a purple and teal skirt than it is to pair a purple skirt and a teal skirt with other items.
- Don't be afraid of your curves. Yes, layering skirts and dresses will add volume to your lower half; sometimes it will add a lot of volume. But keep in mind that if you combine a full lower half with a tailored upper half and good waist definition, the overall effect is that you look smaller. Trust me on this, the only person who cares about the absolute circumference of your hips is you; everyone else just notices your proportions and silhouette.
- A-line skirts work really well as top layers because the flared bottom hem has lots of room to fill up with another piece. But even a fitted dress or skirt can be layered if you're careful. Below are a couple of examples of tight-fitting dresses worn with skirts layered underneath; this is a harder look to make work because of the top layer's propensity to squeeze the lower layer and bunch it up. In the first case it works because the two patterns compliment each other and the bunched skirt looks like a ruffle; in the second case it works because the lower layer is actually a slip and is equally slim-fitting.
- It'll generally be more comfortable if the waistbands of the two layers aren't sitting right on top of one another. I usually adjust the waistband of the top piece to sit slightly above that of the bottom piece. I've also placed an elastic belt on top of the lower skirt and then put the top layer over that; this keeps the bottom layer in place if you need to hike it up or down to achieve the right length. Obviously this is not a good technique for warm days, especially if you have another belt on top of everything (don't laugh; I've done it!).
- Speaking of belts, I find that a belt will help smooth the two waistbands out and will also keep the skirts in place. I very frequently wear a belt when I've got two skirts layered together, but find that I often don't need one for a skirt under a dress.
- Don't forget to take the shape of the two garments into account. If you have a flared skirt for your upper layer, you'll need another flared skirt underneath it or else the layers won't hang together very well. Similarly, a straight skirt or dress usually requires another straight skirt underneath, unless the flare of the lower skirt begins where the upper layer ends, as shown in this example on the right. This bulky tulle skirt is actually only bulky at the bottom panel, so it works fine under a close-fitting tunic such as this one.
So let's hear it -- do you have any additional wisdom for layering skirts? Have you tried this look before?