We say measure but we mean line length. When you read a book, the type measure is the length of the text line, measured in picas.

Well, of course one could imagine a page where the measure isn’t the length of the text line — say this one! — the measure here goes from the left margin (i.e. the start of the word Measure in the title) all the way to the right hand margin (which is pretty much undetectable here as all the lines, being ragged right, unjustified, finish at different points. The two rules above the title show the measure.) So here the text has a line length of say 25 picas, while the full measure is 38 picas. In other words (words that a printer or designer would use in speccing a job) each line of text is indented 13 picas left on a 38 pica measure. (These numbers, while they are more or less an accurate reflection of reality on my iMac, should not be taken as literally accurate. The actual measure you are looking at will depend on how the device you are using is displaying the page.)

It’s for reasons like that explanation that printers have needed the word measure. We don’t do it to confuse: quite the opposite — to make unambiguous cases where confusion might easily arise.

And bear in mind that as with so much in our business the terminology originates in the hot metal/letterpress world. With hot metal type the blank spaces are not the absence of anything: they are the presence of metal spacers which are lower than type height, so that when the type is inked these lower parts receive none of the ink and therefore print blank. You can see the lower parts around the centered lines in this illustration.

Ems and ens is also relevant here.