Once in a while, a formal dress with many layers gets to the store with an uneven hem. This prom dress had 3 layers. The outer layer was polyester chiffon. The second layer and the lining layer were each a lightweight polyester. Together they look nice and create enough fullness to give a swirling look to the dress.
The trouble is, part of the second layer was peeking out from under the chiffon. You can see it here:
When pinning this up on the girl, I try to make the second layer about 1½" shorter than the chiffon. That way if it moves at all, when the girl moves, it will remain under the chiffon layer.
The lining layer (that closest to the body) should even be 1 or 2 inches shorter than that.
Here is what that may look like:
When all is finished, you don't notice how much difference there is between the layers, but you'll notice right away if the underneath layers are too long. Give yourself enough difference between the two so that the dress can shift and flow and still look well done.