Most readers of English have no trouble reading the HOT BEER sign above, even though the letters are all uppercase, generally considered a typography hazard (a practice that risks confusing, alienating, or losing readers). So right away, this HOT BEER sign in this post is an alert that there are many great exceptions to the all-capitalization typography hazard I am writing about in this post.
The Bouma Shape versus ALL CAPS
Named after Dutch psychologist Herman Bouma, the term “bouma shape” was first used in 1997 by Paul Saenger (Space between Words: The Origins of Silent Reading) to describe the shape of a word in multicase letters. The term is still used by typographers and graphic designers as a useful description relating to letterform differences.  The ascenders and descenders of lowercase letters create varied shapes for different words, whereas words in all caps have very little shape distinction since they are all basically rectangles.
Using all caps for headlines, subject lines, and emphasis is a long-established print convention that also works for electronic text. Along with words in bold, words in ALL CAPS call attention to themselves and are often the most important words we want to write, and in addition to color and size, are a way to direct the reader’s eyes to the text we want them to see first or to remember.
So what is the hazard?
Using all caps for whole sentences these days can be read as the equivalent of shouting. For example, something like PLEASE SEND IN YOUR TIMESHEET BEFORE YOU GO HOME can be read as an irritated command as opposed to a simple request, especially when sent in an email. For a brief history of this issue, see Netiquette of Capitalization: How Caps Became Code for Yelling by Alice Robb. 
Using all caps for whole paragraphs can reduce readability, depending, of course, on who the readers are, what the message is intended to accomplish, and how many paragraphs are going to be set. Using all caps for whole paragraphs can also make for some slow reading, and many proofreaders will tell you that text set in uppercase is harder to proofread. So it is no surprise that today we don’t see many newspapers or books set in all uppercase letters. Typographers used to explain this by saying that it is the bouma shape of words that allows for faster reading. However, there are other factors to consider.
Most readers spend the bulk of their time reading multicase text and are therefore more proficient at it. When readers are forced to read large quantities of uppercase text, their reading speed will eventually increase to the rate of lowercase text. Even text oriented as if you were seeing it in a mirror will quickly increase in reading speed with practice (Kolers & Perkins, 1975). — Kevin Larson 
Even so, since readers today read mostly multicase text, for typographers (and writers) setting type, it still makes sense to use all caps sparingly, wisely, and interestingly if we wish to keep our readers. And yet, of the 17 words in the subscribers sign below, 15 are in all caps. Now why does this work? Partly due to good contrast in size and choice of different typefaces. Hm, breaking the oft-quoted type rule of “use only one or two different typeface families.” Nice.
Bouma Shape and How We Recognize Words
The “bouma-shape” model has never been scientifically linked to how readers actually read text. And recently (the last 20 years), cognitive and neurology researchers have rejected the bouma model of word recognition in favor of the parallel letter recognition model, which stresses that the letters within a word are recognized simultaneously, and this is what allows us to recognize the words. This is the theory of word recognition that is most accepted today by psychologists.
For an introduction to the discussion of these issues, including description of eye-movement studies and neural signal processing of reading, see the work of Dehaene, Enneson, Larson, and Sacks, listed below. Much of this work relates to the need to understand the science of how we read and the designing of typefaces, especially for readability and legibility of screen text.
Unpredictable Use of All Caps
In the cat poster shown here, the words in ALL CAPS attract less attention than the bold lowercase due, of course, to larger weight and size of the lowercase letters.
Not Exactly Bouma Evidence. However …
Aoccdring to rscheesrch at an Elingsh uinervtisy, it drosn’t mttaer in waht oerdr the lteters in a wrod are. Thew olny oprmoatnt tihng is taht frist and lst ltteres are in the rghit pacle. The rset can be a mses and you can siltl raed it wouthit a pobrelm. This id bcuseae we don’t raed erveuy lteetr individually, but as parts of familiar wrod shapes. — Alex W. White. Thinking in Type. 2005 
- Saenger, Paul. Space between Words: The Origins of Silent Reading. California: Stanford University Press, 1997.
- Robb, Alice. “Netiquette of Capitalization: How Capital Letters Became Internet Code for Yelling.” New Republic. http://www.newrepublic.com/article/117390 (accessed March 14, 2015).
- Larson, Kevin. The Science of Word Recognition. Advanced Reading Technology, Microsoft Corporation 2004. http://www.microsoft.com/typography/ctfonts/WordRecognition.aspx (accessed 3/8/2015).
- White, Alex W. Thinking in Type: The Practical Philosophy of Typography. New York: Allworth Press, 2005: 14.
- Dehaene, Stanislas. Reading in the Brain: the Science and Evolution of a Human Invention. New York: Viking, 2009.
- Enneson, Peter. “Bouma as Bounded Map.” Typophile. September 29, 2005. http://typophile.com/node/15432 (accessed 3/8/2015).
- Enneson, Peter. “Bouma Slam.” Typophile, December 20, 2010. http://typophile.com/node/77618 (accessed 3/8/2015).
- Sacks, Oliver. The Mind’s Eye. The Mind’s Eye. New York: Random House, 2010.
©2015 Barbara Kristaponis
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are ©2015 Barbara Kristaponis