Speaker arrangement terminology


I use 2 front, 2 side, and 2 rear speakers supplemented by a pair of subs.  Is that considered a 6.1 or 6.2 layout?  If 6.2 and I add a third sub, would that be 6.3?  Specifically, what is the convention regarding the digit after the decimal?  Does it refer to a sub or something else?
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The first number is on the floor plane.

The second is the number of subs

The third number is overhead.

So a

5.2.4 would be 

5 floor speakers

2 subs and

4 overhead speakers

again 

5.4.4

5 on the floor

4 subs and

4 overhead..

so a 

20.10.20 would be a bunch of work.. ;-)
"...The second is the number of subs..."

It’s the number of discrete LFE channels, see below.

I had to think about this (ouch). I think the point one, point two etc., is used in home theater systems with a LFE outputs. A limited frequency band output from the AVR. One point for each channel of LFE. On the other hand in two channel systems with a pair of subwoofers they are really just a separate driver and another "way" of a loudspeaker system. A way being a frequency band of operation. My woofer towers for example can operate outside the band provided by a LFE output. So that system is 2.0 even though the woofers are a separate enclosure. If that system ran pure subs from LFE outputs, it would be 2.2. A least that is the way I see it.

An additional thought; The number of subwoofers does not keep adding additional numbers, it would be the number of LFE channels. One, two, or ten subwoofers run off the same channel is still just point one, same as a single sub with multiple drivers.
Yes, the number after the dot is for subs.  Height speakers like in Dolby Atmos count before the decimal.

However, having said that, this is usually talked about by vendors of equipment.  Like 5.1 or 7.2 receivers. It doesn't make a lot of sense/need for the user.
If you have two subwoofers, but use a Y-splitter cable on one output of your receiver/processor or if you daisy chain one sub to the other, then it is still considered 1 LFE channel since you are only using one output on your receiver/processor.  So, it would be 6.1 if you were using 6 floor channels.

It would only be considered 6.2 if you used two separate outputs on your receiver/processor to drive two separate subwoofers as two individual discrete analog channels.  In this case, the receiver/processor may send certain sounds only to one of the subwoofers, such as bass from the left main speaker/channel if you have that speaker set as "SMALL".  Any LFE effects from the movie (such as earthquakes, explosions) would be sent to both subwoofers.
An output of the Ayre preamp goes to a Velodyne SMS-1 bass manager that provides acoustic room correction for a pair of HGS-15 subs. Each sub is connected via a separate balanced line, but the bass is monaural, so by russ69’s counting my setup is 6.0, with the subs being extensions of the front LR.

For surround, HDMI goes to a Bryston SP3 that returns balanced front LR to the Ayre preamp and thus uses the same subs, so still 6.0. The SP3 does have an unused LFE output, and I have a pair of spare HGS-10s it could feed. I suppose that would then be a 6.1 setup.

My plan is to use a pass through from the SMS-1 to a second SMS-1 with a long balanced line to a third HGS-15 at the other end of the room.
Hmmm. yeah, I would agree with your description of 6.0 and 6.1.

That being said, in your system, I think I would replace the two HGS-15 with your two HGS-10.  Then connect the big HGS-15's to your Bryston .1 subwoofer output.

The reasoning here is that 2-channel music doesn't necessarily need those extra big HGS-15 (unless you listen to stuff like scrillex and NWA, lol).  However, the HGS-15 would really shine on that Bryston's subwoofer output where you get a LOT of low bass on movie's LFE channel.
11.4.4

11 floor speakers

4 subs

4 overhead speakers

AGAIN

9.10.8

9 floor speakers (even the front center speaker is under 4 feet and can be a much larger speaker than the mains) It is actually how HT is suppose to be.. The center speaker is the MAIN speaker... VOICES, especially whispering...

10 subs (location or how it's driven does not matter)

8 overhead speakers (corners and sides BUT on TOP)

The only real choice after that is "Speaker Size" that cuts the mains at a certain point and can add a step up  on the BASS side for the subs.. LFE (step baffle) vs left right full signal. They are a very different way to power subs.. 
I’m sorry but I don’t agree. The first number is the number of channels, 2.0, 5.1, 7.1. It defines the system and the format. Adding speakers does not change the format. So for consistency, the second number should also be the number of channels. The defense rests.... :)
I was asking about conventional usage; I doubt decree applies.  I assume LFE is encoded as a separate channel, but I wonder about counting channels rather than outputs.

auxinput, your suggestion of switching the roles of the HGS-15s and HGS-10s seems rationale.  For stereo, I do enjoy the subtle vibration of pipe organ pedal notes, but IIRC the HGS-10s can deliver that, whereas the HGS-15s can make you feel pressure on your chest with a movie explosion.