Let say we have a list, a sequence or series of things of the same kind (for example a batch of pens). Now we count these things.
- One way to count is that, we take one and count it as the 1st, then the next as the 2nd, then the 3rd and so on. The ordinary numbers here mean that these are counted from one member of the group (the 1st one).
- Another way to count is that, after one we take the next and count it as the 1st, then the 2nd next, then the 3rd next and so on. The ordinary numbers here mean that these are counted after one member of the group (‘the 0th one’).
Now we group the things into groups of each n members (for example 3, 4, 5 or 10 etc.). When we count from the 1st one, the n-th, 2×n-th, 3×n-th, … members are the last one of each group. When we count after the 0th one, the n-th, 2×n-th, 3×n-th, … members are the start one of each group, which contains except for these members the members of the next group in the counting way above.


