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Comment by 13of40 | Hacker News Reader

What are some examples?

There's a German idiom "turning an X into a U" (U being a V) with the origin about changing the number on a chalkboard recording your purchases in a pub. In a hand-written contract you can do the opposite, adding two lines to turn a V into an X.

More generally, a fun fact many miss about Roman numerals is that you can do this with all digits (except I because that would require fractions):

X (10) -> V (5)

C (100) -> L (50)

M (1000) -> D (500)

It's less obvious in prose like this but it helps if you consider that these numbers would normally be drawn with straight lines (e.g. M would often end up looking more like |X| instead of |V|, which explains why cutting it in half would result in |> or D). This is also a handy way of remembering the less frequently used digits like L or D which are harder to remember because they don't really stand for anything (unlike C = centum and M = mille).

If something ends with “I“ you can add two more I’s and have it go from 1 to 3.

And so on.

Using that appending technique works to sneak the year forward in most cases - perhaps that was the intended benefit? We can update the copyright from (say) 1950 to 1951 without any obvious tampering.

This is why you draw a line before, and after amounts on cheques.

V -> VI -> VII

I imagine that the ones with preceeding values would be harder though, perhaps XIX -> XX for example

In handwritten Arabic numerals, you could maybe get away with:

3 -> 8

(non-German) 7 -> 9

(German) 1 -> 4

5 -> 6

Presumably changing I to V or removing prefix subtraction characters (e.g., XIX becomes XX).

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