Box elder is one of the most-often confused plants when compared to poison ivy. Just like poison ivy, it has 3 leaflets, the leaflets are often shaped similar to poison ivy, it can appear shiny, it grows in the same area as poison ivy, it has reddish stems, and the middle leaflet has a longer stem than the side leaflets.
What differentiates it from poison ivy is the leaf arrangement. When you click on the photo below to enlarge it, you will see two sets of leaflets right across from each other, circled in red:
As my identification skills got better, I also noticed a difference in the main stem of box elder. In the photo above, you will see two lighter colored bands around the stem (the bands are not limited to two, that's just all that is showing in this particular photo.) Poison ivy does not have these types of markings.
Poison ivy has alternate leaf arrangement, which means leaflets alternate sides of the stem - they are never right across from each other. There may be a rare occasion when they are pretty close to right across from each other, but that would be one rare spot on the plant, not the entire plant. This is alternate leaf arrangement on poison ivy:
Any time leaves are opposite, you can't be looking at poison ivy. This is a poison ivy leaf circled in orange below. It contains 3 leaflets. Leaf arrangement refers to each group of 3 leaflets (which makes 1 poison ivy leaf). It does not refer to the arrangement of the 3 leaflets to themselves.
So, if a plant displays opposite leaf arrangement, it can safely be ruled out as poison ivy. Also, if you're looking at a plant that looks an awful lot like poison ivy but has opposite leaf arrangement, you're likely looking at box elder.
The original post can be found at http://ohiopoisonivy.blogspot.com/2014/07/what-is-it-wednesday.html