John M. Dlugosz writes:

Will equivalent elements in a ordered_non_unique index be "stable" and maintain their relative ordering based on the order in which they were inserted?

Yes, this is the case, though the explanation is a little sophisticated: Suppose we have an ordered_non_unique index of doubles and the following range happens to be inserted into the container: 1.0 2.0 3.0 3.0 3.0 4.0 5.0 Now we try to insert and element k with value 3.0: will it go righmost or some other place? If we take a look at the relevant piece of code in : bool link_point(key_param_type k,link_info& inf,ordered_non_unique_tag) { node_type* y=header(); node_type* x=root(); bool c=true; while (x){ y=x; c=comp(k,key(x->value())); x=node_type::from_impl(c?x->left():x->right()); } inf.side=c?to_left:to_right; inf.pos=y->impl(); return true; } You can see that k is always compared in a k