As kids, my brothers and I always played around with the ancient beaded calculator, the abacus. But to this day I have never known how people actually use them. To this day when I looked it up. :)
How To Add Two Numbers With An Abacus (via WikiHow):
Say for this example, we're adding 35 to 113.
1. Assign values to the columns in a systematic way from right to left, like here:
This mimics the way that we read numbers; with the highest magnitude as the left-most number, and they decrease to the smallest on the right (like 256 = 2 hundreds, then 5 tens, then 6 ones).
2. Row I beads will be 5 times whatever Row II beads are worth.
3. Start with all the beads towards the ends. Then place the abacus flat on the table.
35 = 3 units of 10, and 5 units of 1
Move the beads towards the center bar accordingly:
3 beads from Row II in the "10s" column
1 bead in Row I in the "1s" column:
5. Break down the second number into units:
113 = 1 unit of 100, 1 unit of 10, and 3 units of 1
Move more beads towards center to account for those units
1 bead from Row II in the "100s" column
1 bead from Row II in the "10s" column
3 beads from Row II in the "1s" column:
If you end up needing 5 or more beads from Row II, you can replace 5 beads from Row II with 1 bead in Row I.
6. Count the beads that are by the center bar and figure out their worth to get the answer.
In this case,
"100s" Column: 1 bead from Row I = 100
"10s" Column: 4 beads from Row I = 40
"1s" Column: 1 bead from Row I, and 3 beads from Row II = 8
If that was too complicated, there's a handy video here. I think the explanation is harder than the practice, actually!
You can imagine this could be as helpful as a calculator for adding many or larger numbers together. Brilliant, considering it's over 800 years old. When people get the hang of it, they can do it very quickly!