Explanation: </span><span>There are two series in this sequence (YBWDUF), which are as follows:
1. The first series is YWU</span>S... in background direction by skipping one letter. Like Y(skipping X)W(skipping V)U(skipping T)<span>S. (Hence S is the next letter)
</span><span>2. The second series is BDF... in forward direction by skipping one letter. Like B(skipping C)D(skipping E)F and so on.
Both of series are in alternate positions as follows: YBWDUF</span><span>S.</span></span>