"And how big an area would 'Monmouth' then have covered ?" http://www.forum.forest-of-dean.net/index.php?id=41651

Hi,
From this & some of your earlier queries I see you have been "confused" as the Forest placenames you're researching, particularly those to the western side nearer Coleford, seem to vary thro the Census years with regard to their location as quoted on Census forms. This subject's been raised a few times, I know I was baffled when I started researching my family in the Lydbrook area just a few miles up the Wye. This was especially difficult to fathom at first as sometimes they appeared to suddenly become Welsh insofar as they were in the Wales Census, or their BMDs were officially registered as "Monmouth", rather than "Westbury On Severn", a place I was far more familiar-with as a resident of the eastern Forest with it being onroute to Gloster and beyond which we often visited, unlike Monmouth way.
We naturally consider the England - Wales border to be the River Wye, the obvious strategic border as used since the Romans and Normans arrived. Monmouth sits on the border/river at the very eastern edge of Wales, yet despite this non-central location is the County town of Monmouthshire which extends westwards into Wales. However despite this the county of Monmouthshire has often been considered an English County, I suspect because of the emergence of it's coal-rich south Wales valleys and associated iron industries....

Monmouth was an important town of local government long before the industrial growth of south Wales, it being considered so as one of the last outposts of "civilised" England before entering the wild and potentially unsafe Celtic country which even the Romans and Normans steered-clear of, just as they did the inner Forest. Monmouth was the area's equivalent of Hereford and Gloucester, hard to imagine nowadays given it's much smaller size. Re local government of the Forest it was equivalent to equally ancient Westbury on Severn on t'other side of Dean. Hence these towns shared the local government duties of the Forest when that started being settled in the early 1800s or so. As the Forest filled with more immigrants thro the later 1800s, and communities and parishes created & grew, so local civic districts also changed in area and name, hence the apparently confusing namechanges between the various Census years. However from a BMD Registration viewpoint, the western area of the Forest around Coleford still tended to come under Monmouth office jurisdiction, later with Chepstow too, so still harking back to medieval times with the castles of Monmouth & St Briavels.

The best way I've found of trying to understand the various civic districts and how they changed thro time is this excellent site, if you still have any queries it should help.
http://www.ukbmd.org.uk/genuki/reg/districts/monmouth.html
As much as I truly love my home area, all this interaction twixt Districts, adjoining counties and countries didn't help my introduction to FH research, so I sympathise for newcomers such as your goodself who don't know the lie of the land either !

If you've not seen it, I often recommend using the Genuki site, it can give a great deal of helpfull information about Victorian life across the UK, such as old Gazetteers, Church Parish info, etc etc.
http://www.genuki.org.uk/big/wal/MON/Monmouth/index.html

http://en.wikipedia.org/wiki/Monmouthshire_(historic)

So it could be argued that "Monmouth" could include the whole County; as I'm sure you know sometimes a Census will show say "Gloster" as a place of birth, whereas in fact GlosterSHIRE was meant. Then again, this entry in a census was purely down to the person being asked, and sometimes they'd give an answer that suits their own views over actual precise locations.
Re the formal BMD registrations, from a Dean viewpoint anywhere in the north-western edge, particularly Coleford area, could come under Monmouth. Then again, I have no idea what (if any) the legal situation was back in the C19th (ie what rules/guidelines there were to ensure a BMD event was reported to the register authority, and by whom), but I've seen cases where these were reported hence registered quite a way from the obvious place of residence - I'd assumed to perhaps avoid embarrassment if the baby's father is in doubt, or the married couple seem rather young or have eloped...

Hoping this helps, keep hunting !