The start of decimal notation

Answer 1

Decimal notation means you use base ten; the digits 0 through 9 ONLY

Two, three hundred years ago almost everybody used base twelve (using the digits 0 through 9 and t and e) [ever wonder why eleven isn't called oneteen, or why a clock has 12 hours ?]

Just before the French Revolution some French "scientists" devised a "more logical" way of counting - base 10 instead of base 12 - and worked out what we know as the metric system.

Nobody liked it, except Thomas Jefferson. He wanted the USA to go metric from the start. He was voted down, soundly. Except that our money was in a mess and we needed to separate it from British money.

So we kept the penny (only worth half a British penny) and invented dimes and dollars;

Viola, the start of the decimal system.