"i before e, except after c".
It's everyone's favourite spelling rule, and it comes in handy every time you want to receive a reprieve after a siege.
But then the rule lets you down when you plan a hiest to sieze some speceis from a wierd athiest sceintist who fanceis you.
So how does the rule live up to reality?
The chart below shows every 'ie' and 'ei' in English, grouped by the letter that comes before it, with the ones that obey the rule in green and the ones that break it in red.
The chart is mostly green, which seems like good news: the rule holds true 81% of the time.
But whose ridiculous idea was it to reverse the 'c' section? 'cie' is much more common than 'cei', so the "except after c" part actually makes the rule worse.
Change the rule to the snappier "i before e, always" and you're up to an 85% success rate.
"i before e, but only after consonants" would take you to 85.5%, but we're into pretty marginal gains at that point.
More details, if you're interested...
You can see the actual numbers in a spreadsheet here.
Source: SOWPODS, the word list for English-language Scrabble in most of the world. If you're from the USA, Canada, Thailand or Israel you might be used to the shorter TWL list. I've put the TWL numbers in another tab of the spreadsheet, and they're about the same.
In the chart, each 'ie' and 'ei' has the same area, which is why the rows have different thicknesses.
A word can appear multiple times: "oneiromancies" breaks both halves of the rule and appears in two red patches.
The bottom row shows the few words that begin 'ei'. There are no words that begin 'ie'.