If you look at them by their botanical names then you can get to the bottom of things. Without mentioning American Elm you have Ulmus rubra ( Slippery Elm, Red Elm, Gray Elm, Soft Elm ) with a sg of .58 and which is the least desirable because the wood is too variable. Then you have Ulmus thomasii ( Cork Elm, Canadian Rock Elm, Rock Elm ) which has a sg of .65, one of the hardest Elm and excellent for bows but hard to find. Ulmus alata ( Winged Elm, Rock Elm, Wahoo ) and with a sg of .69 is the densest of the Elms, doesn't grow up here. Last is Ulmus crassifolia ( Cedar Elm, Southern Rock Elm, Rock Elm, Basket Elm, Red Elm ) which has a sg of .64. Of them all the Cedar Elm is the only one, besides Ulmus rubra, that also has a trade name of Red Elm. All the other species are not native to this country.