Go ask Alice

Yesterday I saw the movie Alice in Wonderland and I noticed the price tag on the Mad Hatter's hat said "10/6".  I recognized that it was a price in old English currency of ten shillings and six pence.  Since there were twenty shillings in a pound and twelve pence in a shilling, 10s 6d would today be equivalent to 52½p or £0.525, although halfpennies stop being minted in 1984.

That got me thinking of duodecimal measures.  The words "inch" and "ounce" both come from the Latin uncia, meaning a twelfth part, and there are twelve inches in a foot, and twelve troy ounces in a troy pound, although other types of ounces no longer represent a twelfth of anything.

Years are divided into twelve months.  Originally sun dials divided the period of daylight into twelve hours, which varied in length depending on location and time of year, but eventually mechanical clocks came to represent periods between midday (noon) and midnight, and these clocks are divided into twelve hours of equal length.  An additional hand likewise divides each hour into twelve parts; adding marks between the numerals multiplies this division by five, giving 60 minutes, but have you ever noticed that the time is almost always rounded to the nearest multiple of five minutes, so we're still dividing hours by twelve?

A dozenal system of units would be fine, and in some ways better than decimal, but traditionally there have been only a handful of dozenal units.  A shilling were divided into twelve pence, but a penny was divided into two ha'pennies or four farthings, a pound into twenty shillings, etc.  Since Decimal Day, 1971 February 15 (MJD 40997), a pound is divided into 100p, period.

MJD 55269.198

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