Definition of bilateral
Bilateral" means that the print can be touched from both sides, from above and from below. If this is still not perfectly clear to you, nothing better than an example. You receive 4-5 preflop. If the flop falls 6-7-x or 2-3-x, you will have a two-sided straight draw. In the first case, a 3 will give you the big straight and an 8 will give you the small straight. In the second example, same principle, you can complete your small straight with the 5 or your big straight with the ace.
The importance of distinguishing between small and large fifths
In our example above, we say that with 4-5 over 6-7-x, we would have a big straight if a 3 fell on the turn and a small straight if an 8 fell on the turn. The reason being that the big straight will give us the best possible straight. On 6-7-x-3, the best straight is made with 4-5. But if you have 4-5 on 6-7-x-8, your straight would be beaten by 5-9 or 9-10. The two-sided draw increases our chances of completing it. With 4-5 over 2-6-J you have 4 cards that could give you a straight (all four 3s for a non-bilateral draw) whereas if the draw is bilateral as on 5-6-x, you will be helped by 8 cards!
