I am now retired. I remember a thousands of an inch was often called a
'thou'. Is it true it is also called a 'mil'?
Anybody knows the background to the usage of these two terms. Was one more
British and another one more American. And is the term 'thou' still used?
The terms seem to have found homes in different industries. If you
look at the fine print on a package of plastic garbage bags, you'll find
that the thickness of the plastic is typically expressed in mils. That
seems to be the standard for that business.
But if you walk around a machine shop, you'll most often hear people
talking about "thousandths", "thou", or "tenths". Tenths, of course,
are ten-thousandths of an inch.
You'll also hear references to microns, or U's (pronounced "youse"),
from the U-like appearance of the Greek letter omicron. Those are the
standard terms for thousandths of a millimeter, or millionths of a
meter. As the US continues to move slowly toward total metrification,
and as manufacturing processes become more and more capable of dealing
with micron-level precision, I suspect that youse will become more and
more common around many shops.
Which ought to cause a great deal of confusion, and maybe even some
interesting humor, here in Philadelphia.
"Mil" is usually used to express the thickness of a film or coating.
"Thou" is simply a slang abbreviation for "thousandth", or "thousandth of an
inch" and is very common (US). In contrast to "mil", "thousandths" can be
used to refer to much larger dimensions. For example, a machinist would
probably refer to 0.100 in. as "one hundred thousandths", or "a hundred
thou". Mils tend to be restricted to small single- or perhaps double-digit
The Greek letter omicron doesn't look like a "u", it looks like an "o".
Microns are represented by the symbol for the metric prefix "micro-" which
is the Greek letter "mu" (µ). It is often represented as a small "u" since
mu is not easily typed on many keyboards (particularly the old typewriters).
This would make sense as "milli" means one thousandth, but I find that
today (in Britain) the word "thou" generally refers to a thousandth of
an inch and "mil" to one millimetre.
The influence of metrication I guess.
I knew that. I don't have a clue about why I typed omicron. Of
course, there ARE some days when I can't tell the difference between my
ohmmeter and my micrometer. They both measure something in Greek, right?
Thanks for waking me up, this morning.
Thanks to all of you for your quick comments.
I have been brought up years ago using 'thou' to stand for a thousandth of
an inch, and I now understand it is still very much in use in conversations
in workshops and engineering offices.
I notice 'mil' is used mainly in thin film and coating industries to refer
to the same measurement. On the other hand, it would cause grave
misunderstanding, would it, if as one of you said it is also used in
conversations to refer to one mm ?
To sum up this greek vocabulary issue, I would like to add that the reason
that the word "omicron" has the part -micron, is because "micro"
means "small" in greek and the letter omicron means "small o" while the
letter omega means "big o" and in anciend greek there was a difference in
pronunciation, omicron being a shorter o than omega. Now there is no
difference in pronounciation. You can also notice that "Megabyte" for example
also uses the greek word "mega" (=big).
By the way, 10^-9 m are called nm which stands for nanometers. "Nanos" in
greek means dwarf. And "gigas" (e.g. gigawatt) means giant.
Sorry if this comment was irrelevant.
"Kirk Gordon" wrote
Do you mean the abbreviation of a micron is not pronounced in US industries
as "miu" ? This is the pronounciation of the Greek letter that represents a
micron. It looks like u with a long leg in front.
About your last point, it is all right if these only cause confusion. What
if they end up as mistakes in works to be done ?
Don Gilmore caught my error on the first point. It's the Greek
letter mu that looks like an English "u" with a tail on it. That's why
some American machinists refer to microns as U's. Regarding your second
question: I can only assume that youse have never spent much time in
Philadelphia. Here, youse'll hear the word "youse" (pronounced yooooz)
used quite frequently. In a machine shop that makes, say, precision
parts for universal joints, a sentence like "Youse gotta take a couple
more u's offa dem U's, or we ain't gonna be able to use 'em." would make
If I were to use a really small (micro-sized) micrometer to measure
the size, in millionths of a micron, of a certain sub-atomic particle,
would I then have a micro-micro-micro-mu-mu-meson measurement?