Here are a few questions I have about PL lamps:
1. What does PL mean?
2. Where do the unimaginative names for PL sockets come from?
3. What are the different types of PL lamps?
4. How does PL13 work? You need 2 pins for each electrode, but there are only 2 pins in total for the 2 electrodes, and that would make sense if was a CCFL or Instant Start, but it's preheat!
5. How can they fit such high power into a tiny amount of area? This only happens on the bigger types.

Socket names loosely based on :

x - If this lamp base comes always with 4 "main" pins in a row, then it is considered as 2 bases of 2 pins

G - pin base
GX - second (incompatible) option of the pin base, usually with double springs

xx - distance between 2 "main" pins in mm

d/q - for lamps where the 2 "main" pins are diagonally from each other, whether the base is double pin (d) or quad pin (q)

-x - keying options

So for example :

G23 - PL9 base
GX23 - PL13 (US) base
2G7 - PL9 4pin base (4 pins in a row, all "main", with 7mm distance between 1-2 2-3 3-4, so its considered as "2 2-pin 7mm bases")
2G11 - PLLx 4pin base
G24d-1 - PL13 (EU) base
G24d-2 - PL18 base
G24d-3 - PL26 base
G24q-3 - PL26 4pin base
GX24q-3 - PLT26 4pin base

sorta



http://www.osram.com/osram_com/products/lamps/compact-fluorescent-lamps-without-integrated-control-gear/index.jsp - European versions (Few lamps are different for US)



The starter is inside, permanently connected to the other 2 pins. There are PL lamps with 4 pins outside, some come in either type, some others (like PL-L) only with 4 pins



The Mercury discharge can handle it, the Phosphors (Triphosphors) were specifically developed for those compact lamps (also 2D and the 1st CFLs) in the 70s/80s, as well as other design detail to prevent it from wearing out at the higher loading and smaller lamp volume (so higher vulnerability to contamination)

I almost forgot that the 2 pin PL lamps have a starter inside, but I need a diagram/schematic to understand.

No different than how you'd wire Preheat with a F20T12. The lamp+starter are enclosed in one package, the 2 remaining pins coming out

No different than how you'd wire Preheat with a F20T12. The lamp+starter are enclosed in one package, the 2 remaining pins coming out

But I'm still confused. I have never seen a good diagram of how preheat ballasts are wired.

Now I get it.

Those circuits are for 230V. On 120V some things are different :

 - No tandem circuits with 2 wire ballast (except for 4W T5, but there are no suitable starters for 2x4W T5 setup)

 - "High reactance transformer" = HX ballast. For 2 20W lamps it can indeed be wired like that with a 1x40W 120V Preheat ballast. 2x40W 120V Preheat ballasts are made with a ballast that have 3 main coils (not counting the starting compensation 4th coil), not pictured

 - The series capacitor can be used with any of the circuits. In case of the HX ballast, it is placed on the lamp side in series with 1 of the lamps

Those circuits are for 230V. On 120V some things are different :

 - No tandem circuits with 2 wire ballast (except for 4W T5, but there are no suitable starters for 2x4W T5 setup)

 - "High reactance transformer" = HX ballast. For 2 20W lamps it can indeed be wired like that with a 1x40W 120V Preheat ballast. 2x40W 120V Preheat ballasts are made with a ballast that have 3 main coils (not counting the starting compensation 4th coil), not pictured

 - The series capacitor can be used with any of the circuits. In case of the HX ballast, it is placed on the lamp side in series with 1 of the lamps

I was thinking of the top left circuit, which is the basic one, and the most likely situation for PL lamps.

I was thinking of the top left circuit, which is the basic one, and the most likely situation for PL lamps.

That is exactly the one for the PL lamps.
You may use a capacitor in series (as the last picture, but with just single lamp), that would make sense when two lamps used in a pair to suppress the 120Hz flicker.
The "PLxxx" comes from the dutch (read: Philips) lamp identification system...

I once heard of ProLight lamps and adapters. Is this what PL means?

I once heard of ProLight lamps and adapters. Is this what PL means?

No.
The "L" means fluorescent, the "P" I do not know exactly (either it means compact, multiple tubes or one socket or something like this)
Regular tubular fluorescents are designated as "TLxx"

I'm a fan of pl-lamps, but I've been long wondering if it's just my imagination or is pl-lamps more prone to show mains flickering as I have noted that they would be?

It appers to be ballast dependant. I see the flicker sometimes with some cheap ballasts, but no flicker with several Eltam and VS (Israel 90s/2000s) and one Vatra (USSR 80s) ballasts i have in exeryday use

Magnetic ballasts are guaranteed to flicker.