Looking at some of my old schematics, I can see that I haven't been entirely consistent when naming components. Nowhere near consistent, actually.
Sometimes I have used "T" as prefix for transistors and sometimes "Q". And especially IC's that are prefixed either with "U" or "IC" – often I have used both in a single schematic. And also connectors that are named "CON" or "J" or "JP". Sloppy…
Well, no more! This page on Wikipedia has a nice table with designators for various component types (even though it does not comply with with IEEE- or ANSI-whatever standard).
So this is what I will use henceforth:
|D||Diodes – all types including Zener, Schottky and LEDs|
|U||IC's of all kinds. Technically it is used for "Inseparable Assemblies".|
|J||Connectors. The more fixed part of a connector pair (often the part soldered to the PCB).|
|P||Plugs. The less fixed part of a connector pair (i.e. the one on the wire).|
|Q||Transistors – all types including MOSFETs, IGBTs, BJTs etc.|
|Y||Crystals and oscillators|
|X||Transducers/sensors (no, X is not for crystals anymore).|
|JP||Jumpers. Not connectors but only actual jumpers that are linked or unlinked.|
|TP||Test point (pad or pin for measuring)|
|MP||Mechanical parts (standoffs, screws, mounting hardware etc.)|