President Donald Trump on Sunday night threaten Iranian President Hassan Rouhani on Twitter, in all caps.

To understand why President Donald Trump decided at 11:24 p.m. Sunday night to threaten Iranian President Hassan Rouhani in ALL CAPITAL LETTERS, you have to go back to AT LEAST 1880 and the publication of a book titled "The Standard speaker and elocutionist."

In the preface, author John William Kirton, a writer of fiction and advice books, wrote "that in the Science of Elocution there are rules which must be observed, and laws which must be obeyed, if perfection is to be attained, and success assured."

The rule that seems to enlighten the Trump ALL CAPS matter appears on page 188 and is titled, "SHOUTING STYLE." It says: "This will be seldom needed throughout an entire piece, but wherever the words imply calling, or commanding, it will be in keeping with the words to employ it."

Several examples are then cited.

One is: "UP DRAWBRIDGE, GROOMS! WHAT, WARDER, HO! LET THE PORTCULLIS FALL."

Another is: "TO ARMS! THEY COME! THE GREEK! THE GREEK!"

It is NOT KNOWN whether the president studied "The Standard speaker and elocutionist" prior to tweeting at ROUHANI but there was really no need, for this style of writing - SHOUTING AT THE READER BY HITTING THE CAPS LOCK BUTTON AND THEN LETTING IT RIP - has been a function of the printed word since before Kirton EVEN PUT IT INTO WORDS.

Tech journalist Glenn Fleishman, the author of a discursive history of TYPE AND LANGUAGE titled "Not to Put Too Fine a Point on It," seems to have found one of the earliest examples of "SHOUTING WITH CAPITALS," as he calls it, in the Feb. 28, 1856 edition Washington Evening Star newspaper.

"The ALL CAPS appears in a story about a Dutch fellow who contracted small pox and is quoted as saying, quite phonetically:

" 'I dells you I've got der small pox. Ton't you vetsheh? der SMALL POX.'

"The author/narrator adds:

" 'This time he shouted it out in capital letters.' "

So literally for centuries, from the days of small pox to Twitter, humans have been using ALL CAPS to try and communicate VERY IMPORTANT points or commands in the same way a child might be told, "IF YOU DON'T CLEAN UP YOUR ROOM I AM GOING TO DISPOSE OF THE CONTENTS ON THE FRONT LAWN."

In so doing, they have REALLY ANNOYED people.

And there have been consequences.

Take, for example, this news item from the New Zealand Herald in 2009:

An Auckland accountant was sacked for sending "confrontational" emails with words in red, in bold and in capital letters.

FIRED.

But the broader consequence, according to numerous academic studies, is that by using ALL CAPS, the writers who employ this TACTIC OF COMMUNICATION aren't having the effect they might believe.

WHY?

BECAUSE ALL CAPS IS NOT AN EFFECTIVE FORM OF COMMUNICATION.

(Whoops.)

Studies show that writing in ALL CAPS causes readers to retain less information, to skim over the words, and to, in general, become confused because it's REALLY DIFFICULT to make out the shapes of LETTERS when they are all VERY LARGE.

"In text, capitals take about 12 percent longer to read than lower case setting," a reported a 1979 study titled, "Why is lower case better?"

Newspapers discovered it was better for headlines in the 1960s after a series of studies, including one in the Journal of Applied Psychology, which concluded in ALL CAPS that "WHEN MOST OF THE MAIN HEADLINES WERE PRINTED IN LOWER-CASE LETTERS, THE SUBSIDIARY HEADLINES WERE MORE LIKELY TO ATTRACT ATTENTION WHEN THEY WERE PRINTED IN CAPITALS THAN WHEN PRINTED IN LOWER-CASE LETTERS OF THE SAME POINT SIZE."

So why is the headline on this story written in ALL CAPS?

Because the caps lock button is unbearably tempting.

MERE MORTALS CANNOT KEEP THEIR FINGERS AWAY.