The decimal point protects the right edge of the amount, while the dollar sign protects the left edge.

Similarly, when people write out a number in English on a check, they often write "Twenty dollars only", not just "Twenty dollars", in order to prevent other words from being added to the end, e.g. "Twenty dollars and a hundred". (There's a nursery rhyme that goes "four and twenty blackbirds", so writing the smaller number first is clearly not unheard of.)

(Except in Belgium and Switzerland and this-one-valley-in-Italy et cetera, where it is just "nonante-trois".)

Compared to that, "drei-und-neunzig" is still not bad.

This is why you fill the empty part of the field with a long horizontal line.

Was this really a problem at some point? My friends have told me about it, but my parents never bothered to mention it to me growing up.

Luckily, though, it is common to use commas to separate groups of three digits: $200,000.00 But this means a change of a factor 1000 which might be less useful than the original 20$ -> 320$ example.

If I have a legal document where I need to be certain of its provenance after going out into the world, it is a digitally signed PDF.

If I print a paper check where the numbers need to be tamper-resistant against casual forging, I use a secure number font. I don't anticipate ever being in a position where my adversary in that use case is a determined, professional forger. I'd insist upon encrypted and/or insured electronic transactions, like through SWIFT or ACH, before I get to the point I'd be worried about professional forgers.

In Chinese, some characters representing numbers are so trivially forgible by adding a stroke or two (1 = 一, 2 = 二, 3 = 三, 10 = 十, 1000 = 千) that accountants came up with complicated substitutes that cannot be altered to mean any other number (1 = 壹, 2 = 贰, 3 = 叁, 10 = 拾, 1000 = 仟). The substitutes are still widely used in legal and financial documents.

eg http://www.independent.co.uk/news/business/news/facebooks-ma...