I am curious as to if anyone knows of a diagram or resource that explains what a Pica is, in relation to points and pixels and such.
It might be something interesting to create for educational purposes and for myself!
1 point × 12 = 1 pica
1 pica × 6 = 1 inch
1 inch × 12 = 1 foot
1 foot x 3 = 1 yard
1 inch = 2,54 cm
There is a diagram about English length measurement system [[http://upload.wikimedia.org/wikipedia/commons/c/c8/English_length_units_...|here]].
12 points equals a pica, but 6 picas (72 points) only equals an inch because everyone agrees that it does. If you extrapolate 12, 6 pica inches, they do not equal a foot. They fall a little short. Everyone who owns a pica rule knows this. Admittedly not that many people own pica rulers these days, but there you have it.
Well, there is the traditional pica and the PostScript pica. Six PostScript picas = one inch, but six traditional picas = 0.996264 inches.
Pixels are a relative measurement and are dependent on device (screen, printer, imagesetter, whatever) resolution. On a 72 dpi device, a pixel = 1/72 of an inch. But on an iPhone 4 (for example), a pixel = 1/326 of an inch.
The topic of pixels continues to become more complex, as hi-res devices (like iPhone 4) no longer map CSS pixels (a.k.a "logical" pixels) at a one-to-one correspondence with physical device pixels. This article explains more on the topic. So, to clarify Mark's statement, 1 physical device pixel on the iPhone 4's screen = 1/326 of an inch.
While a pica is 12 points, nominally 1/6 of an inch (but the hot metal point is not actually 1/72 inch exactly) there is a way in which the term "pica" is used that is slightly more complicated than that which does need a real explanation for a newcomer.
If you are looking at a column of body copy, 1/6 of an inch vertically will be termed a "pica".
However, 1/6 of an inch horizontally will be called a "pica em" instead.
For any point size of type, an "em space" is one that is square in shape, while an "en space" is one half as wide as a square. If one wishes to measure horizontal distances in body copy in absolute terms, rather than relative to the type size in use, then instead of just saying how many ems occupy the distance, one has to first specify the type size and then multiply by the em, as, in printing, vertical and horizontal distances are measured by different units.
It should also be noted that in some foundry types, a 7 1/2 point en space might not be exactly 3 3/4 points in width - for easier setting of tabular matter, nominal en spaces (and, much more likely, nominal 3-to-em and 4-to-em and 5-to-em spaces) could well be changed in their widths so as to occupy an integral number of 1/2 points. This is described in some ATF catalogs.
With Monotype equipment, one might have a typeface which is, say, 11 point and 10 1/2 set, which means that the height of the type is 11 points, but the widths in the system of 18 units to the em are as if the type was 10 1/2 points.
Then, there's the fact that while the Selectric Composer had a point of 1/72 inch, the foundry point is 0.013837 inches, the Linotype point is 0.014 inches (or maybe not; I had read that somewhere, but lately I've read other things that contradict this)... and, of course, the Didone is roughly 15/14 of a point... and 12 Didot points equal a Cicero, the equivalent of the pica in that system.