Let's see an example of nested loops. It's very common when dealing with algorithms to have to iterate on a sequence using two placeholders. The first one runs through the whole sequence, left to right. The second one as well, but it starts from the first one, instead of 0. The concept is that of testing all pairs without duplication. Let's see the classical for loop equivalent:

# pairs.for.loop.py
items = 'ABCD'
pairs = []

for a in range(len(items)):
    for b in range(a, len(items)):
        pairs.append((items[a], items[b]))

If you print pairs at the end, you get:

$ python pairs.for.loop.py
[('A', 'A'), ('A', 'B'), ('A', 'C'), ('A', 'D'), ('B', 'B'), ('B', 'C'), ('B', 'D'), ('C', 'C'), ('C', 'D'), ('D', 'D')]

All the tuples with the same letter are those where b is at the same position as a. Now, let's see how we can translate this in a list comprehension:

# pairs.list.comprehension.py
items = 'ABCD'
pairs = [(items[a], items[b])
    for a in range(len(items)) for b in range(a, len(items))]

This version is just two lines long and achieves the same result. Notice that in this particular case, because the for loop over b has a dependency on a, it must follow the for loop over a in the comprehension. If you swap them around, you'll get a name error.