Helping The Photoshop Help Blog, and why designers use that weird Pica measurement system (to save a whole lot of math).

On the Photoshop Help Blog a reader, Erin, says she has been given the task of creating a Photoshop document that measures 125p x 165p. Her question to the Photoshop Help Blog is whether the "p" in the dimensions means pixels, which creates an image much smaller than she would have expected.

Jim Barthman, the proprietor of the Photoshop Help Blog, replies: "I vote with you that 'p' stands for pixels. However, when it comes to digital graphics, small is a relative term… Feel free to leave me a comment and let me know more about this curious problem. Who is requesting the size? That question might clear everything up."

Actually, the “p” is shorthand for pica, the standard unit of measurement used in graphic design, desktop publishing, and other design and printing industries.

A pica is 1/6th (or 0.166) of an inch. Dimensions of less than full pica increments are measured in points, which should be familiar to anyone who types on a computer.

When typing text in any computer program from InDesign all the way down to Microsoft Word or even WordPad, the size of type is set in points. Since points are increments of pica—just as inches are increments of feet—every computer user works with pica, though most do not realize it.

There are 12 points in a pica, thus the default size of type in InDesign, QuarkXPress, Word, and, of course, Photoshop, is 1 pica.

So why use pica instead of pixels, inches, or even millimeters?

Pixels is a measurement system that was created—and is ideal—specifically and solely for designs that will never be printed, such as for web sites. The system has no relevancy to print. In fact, pixels are not so much a measurement system as a unit of resolution. The Macintosh has a screen resolution of 72 dots/pixels per inch (”dpi” or “ppi”), and Windows 96 ppi. On the Web, the standard for digital image resolution is 72 ppi. An image that is 144 x 144 pixels displays as 2 inches square, though inches are irrelevant on screen or on the Web.

On a computer screen, the Web, or digital camera, pixels are the constant; every image is displayed as 72 or 96 pixels. This is resolution, how many pixels fit within an inch of screen space, thus how sharp is the image. Since the Mac is fixed at 72 ppi and Windows is similarly fixed at 96 ppi, the resolution of the display is fixed and constant; every inch of a Mac display has 72 rows of pixels and 72 columns of pixels. Thus increasing or decreasing the measurements of an image in Photoshop (or anywhere) by changing its pixel dimensions changes is resolution, not its physical scale. The physical dimensions of an image, how wide and how deep, is measured in pica, inches, millimeters, or another system.

Conversely, altering the pixels per inch, where the pixel measurements of an image remain constant will change the physical dimensions of the image because ppi changes the inch part of pixels per inch. More or fewer pixels are being fit into the same amount of space, thus the physical dimensions of the image are being reduced proportionately. If the 144 x 144 pixels does not change as the resolution is increased from 72 to 144 pixels per inch, the image is told to hold twice as many of the image’s pixels in the same space; the physical measurements will reduce from two inches square to one.

Novice designers and even many veteran professionals in America do prefer inches—usually inches decimal so 1 1/16th inches becomes the easier to work with 1.0625 inches. Throughout the rest of the world, the minority of professional creatives who choose not to use pica employ the metric system’s centimeters and millimeters. It should also be noted that some areas have yet other measurement standards, such as didot or agate, though pica is the most common.

Pica is the standard because it works. Pica is used because it is the standard; in this digital, post-Desktop Publishing Revolution world, it is still the standard because it is used. You already use pica, you just probably do not recognize that you use it.

Type is measured in points, a division of pica. So is leading, the vertical spacing on which type sits. Working with type and leading in points and all other elements of a design in some other measurement system like inches increases the difficulty of determining and creating vertical, and often horizontal, measurements and spacial arrangements tenfold. The designer must do the math and conversion from one system to the other, or rely on her imprecise eye.

When assembling out a multi-column layout for print such as a newspaper, magazine, or brochure, it is important that lines of text line up vertically across columns; line 52 in the first column must align vertically with line 52 in the second, third, and fourth columns for clean, professional results and layout balance. This is not difficult to achieve with a text-only layout wherein all columns contain exactly the same number of lines of text, all at the same type and leading sizes. But, introduce a picture or illustration that causes the text of one or two columns to wrap (or “runaround” as some applications call it), and inches or millimeters suddenly become a minefield. Every movement, every nudge of the picture or illustration blows out the column alignment like a bouncing betty.

Just consider the time lost to tweaking the image size and text wrap (or runaround) outset frame to restore cross-column vertical alignment to wrapped text.

Here is an example: Imagine a three-column layout where the type is set close to the ideal at 10 points on 12 points (”pt” or “pts”) leading; the ideal is actually, depending on the typeface chosen, somewhere between 9.5 and 10.5 pt on 11.5-13 pts, but for this example the simpler the better. An image measuring the wide of the column and 1.5 inches deep (tall) is inserted in column one. With inches decimal as the measurement system employed, setting a wrap outset (the distance beyond the image borders inside which text cannot appear) of 0.15 inches on the top and bottom becomes tempting. But just look what happens to the alignment of text across columns! The same thing happens at the bottom of columns, making their ends ragged (uneven). Terrible!

Lines do not match up across columns.

 

Trying a different outset, perhap