later, people realized that a 5.11 climb is much harder than a 5.10. because of this, many climbs were classified as 5.10s, and some were much harder than others. to fix this problem, the system divided 5.10, 5.11, 5.12, and 5.13 climbs even further. they did this by adding the letters "a," "b," "c," and "d" to each grade. within each grade, "a" is the easiest and "d" is the hardest. for example, a 5.10a is the easiest possible 5.10 climb. a 5.10d is the hardest possible 5.10 climb.