no IC but U no T but Q

[Simon]

I've wondered for a long time why in schematics IC's are always marked as U followed by a number when I'm sure it used to be IC which makes sense and transistors were always T but now are Q.

Can anyone think of any others like this ?

[Zero999]

U makes sense to me because I see it as standing for Unit.

I don't know about transistors. I think Q is an Americanism. I prefer to use Tr rather than T which could mean transformer, although I admit to using Q also because it's the default for some of the schematic programs I use.

I don't know why L is used for inductance and I for current. I suppose C can't be used for current because it can easily be confused with capacitance and I can't be used for inductance because it would be confused with current.

[joelby]

L for inductance is named after Heinrich Lenz.
I for current is the abbreviation of the French, intensité.
Q for transistor is due to the quiescent (Q) point.
U, I'm not sure, but 'unit' sounds like a good guess.

[mikeselectricstuff]

I suspect it's just a case of what letters are left. You want a single character to save space on a silkscreen, and I is undesirable as it looks like a one.
Transformers predated transistors which would explain Q as a probably fairly arbitary choice.

Off the top of my head I've seen :

A ?Antenna
B Battery, Bridge rectifier, pushbutton
C cap
CR diode (Crystal rectifier)
D diode
E
F fuse
G 
H
I Undesirable as it looks like one
J Jack
K Relay (clicKy thing?)
L Inductor
LP lamp
M Meter, motor, MOV
N
O Undesirable as it looks like zero
P Pot or plug/pin header
Q Transistor
R resistor
RV variable resistor
S Switch (or SW)
T transformer, thermistor
U IC
V Valve/Gasfilled tube e.g. neon, varistor
VR variable resistor
W wire link
X Crystal
Y Crystal
Z Zener

[Simon]

so maybe it is something dictated by the fact that more of us can make our own boards and it starts backwards from the fact that you don't want to use something like IC in a schematic that will then transfer to the layout with IC being easily misread

[Bored@Work]

They are standardized in IEC 61346 and previously in IEC 750. Only that especially in the US many don't give a thing about these standards.

[KennethCraig]

Here's what Wikipedia has to say... it seems to be the common usage though, but it links to the spec: http://en.wikipedia.org/wiki/Reference_designator

[Zero999]

I couldn't find a complete standard on the Internet without having to pay but I have a hunch that Q for transistor won't be in the standard. In fact I doubt the letter Q will be used at all because it looks too much like O with a grain of dust on the paper.

I think that all standards should be made available for free or a nominal fee to cover the serve cost or printing and allow redistribution for free. I get the feeling that the IEE just update their standards, in order to make money. If they want standards to be widely used then they should be easily available, which is why they should be free.

[Simon]

It's the same with mechanical standards Hero, I had a problem with a plug gauge or rather the suppliers thread and needed to make sure of the full gauge spec and that it was correct for that thread, the only way being to refer to the BS standard, which we do not so we had to ask the supplier to check and they had to wait for "the" guy with the access to look up the standard.