Counting and Recording Numbers

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In a unary system, one mark or notch represents one actual thing.
So /// is used to represent the number three.
In more recent times, statisticians use tally marks.
Every fifth stroke is drawn through the previous four.

About 2500 years ago the Ancient Romans took it further.
They gave every fifth notch an extra cut /\ (still standing for one).
Later the notch marks /\ were written as a single letter V.
And the / was written as I.

So 8 went from //////// to /////\ /// then IIIIV III.
If they were recording an answer they simply wrote 8 as VIII.

Similarly, every tenth notch was given a second cut to become X.
So the notches for 12 would be ////V ////X //.
To record 12 they simply wrote XII.