Fifth Imperium
Most of those far-flung eras have their own, corresponding locales. The Interstellar Wars are set mainly within the Solomani Rim, Milieu 0 centers upon the Sylean stars, and the Gateway Era focuses on the Gateway Domain. However, once your approach the best-developed Traveller campaign settings--starting with the Golden Age that was the basis for both Classic Traveller and Mongoose Traveller--you'll find that there are a wide variety of different settings you can use, spanning the length and breadth of the Third Imperium ... and lands beyond.
An Overview of Space
Space in the Traveller universe is broadly divided in three different ways.
First, you have clusters, which are a relatively new geographical notation, mainly pushed by Martin Dougherty through the T20, Classic Traveller, and Mongoose Traveller books published by Avenger Enterprises. A cluster is a set of 10 or 20 stars that are bound together by politics and/or by geographical closeness.
Second, you have sectors, which are an official geographical designation within the Third Imperium. They encompass all of the stars within a grid that's 32x40 parsecs (and which includes sixteen 8x10 subsectors). Going back to Classic Traveller days, campaigns were typically defined by a sector.
Third, you have domains, which are also official Third Imperium geographical designations, and which include four sectors arranged in a square. They're ruled over by an Arch Duke. In the MegaTraveller era, DGP started to write about the Domain of Deneb, an area which included the Spinward Marches, the Deneb Sector, the Trojan Reach(es) sector, and the Reft sector. Some more recent books have also detailed four-sector areas.
For the purposes of this article, I'm going to mainly talk about sectors, as that's overall the most frequent size for a campaign setting, though you'll see one domain and a number of well-detailed sectors that have less well-detailed sectors nearby.
With that said, here's a look at the many sectors that you can set your campaigns in. In this article, I'm going to cover the first half of them, from Core (at the center of the Imperium) to the Hinterworlds (on one of its frontiers).
If you'd like detailed information on the stars of the sectors, I'll point you to The Traveller Map, a superb resource that we only could have dreamed of back in the 1980s, when I was last an active Traveller fan.
Traveller Sectors: Core to the Hinterworlds
Core Sector 
. The Core Sector could offer an interesting basis for a very political campaign, as it's the center of the Imperium. As we'll see, it's also one of just a few campaign settings that lies toward the interior of the Imperium rather than sitting upon a frontier.
Unfortunately, the material detailing Core is pretty limited. All of the Milieu 0 material for Traveller 4 (aka Marc Miller's Traveller) is set in the area, but some of it's pretty bad (other, perhaps, then the main book, Milieu 0)--and it is 1000 years out of date if you want to to run a Mongoose-era campaign. However, you could supplement the T4 material with information from issues #9-10 of The Traveller's Digest and maybe also with some of the books and articles detailing Imperium nobles.
Generally, however, Core is one of the weaker settings that I'll be suggesting in these articles, and it's included mainly because of the information produced in the T4 days.
Diaspora Sector . During the first part of the MegaTraveller era, GDW flailed around a lot without establishing a base campaign setting. There was a half-hearted attempt to introduce a sector in Challenge magazine in 1989 (the Hinterworlds, which I'll return to at the end of this article), but it wasn't until 1992, five years after the release of MegaTraveller, that the game got a home base: the Diaspora sector.
Diaspora lies just coreward of the Solomani Rim and just spinward of the Old Expanses, two other sectors that I'll be discussing in this article series. Thus, it lies in good relation to a few other Traveller game settings. It's also almost entirely the vision of one author, Charles E. Gannon, who not only wrote the primary sourcebook for the area, but also wrote many adventures and other articles in the setting, for both Challenge and Traveller Chronicle magazines.
However, other than that those plusses, I don't find Diaspora a very good locale for most eras of Traveller play. Firstly, that's because it's a pretty central sector of the Imperium--and unlike Core, it doesn't have a lot generally going on. That means it's not very interesting until the Rebellion starts breaking the Imperium apart, and that's probably out of scope for your Mongoose campaign. Second, almost all of the description of the sector comes from sourcebooks set in the Hard Times era, which is markedly different than how the sector would have been in the Golden Age, before the Rebellion caused it to shatter.
If you're nonetheless interested in the sector, you can find the best information in Astrogators' Guide to the Diaspora Sector and in Traveller Chronicle #2-6 (though Traveller Chronicle, which I'll be mentioning throughout this series, as it's generally a great source for setting material, is relatively hard and/or expensive to collect; I'm still missing a couple of issues from my own set).
I should also note that Diaspora (and Old Expanses, which I'll talk about in the next article) was the main setting for Traveller: The New Era, though things have changed so much by then that the the data for that era is largely useless for any other play.
And now, having talking about two sectors that are pretty outside the norm for Traveller campaigns, I'm going to plunging into the frontiers more typical for the game ...
Far Frontiers Sector . In the days of yore there was an event called the Great Land Grant. This was when a very young GDW supported its early licensees by giving them sectors in the Official Traveller Universe to play with. We'll meet a number of these land grants over the course of this article series. One of them was the Far Frontiers sector, which was the official sector of FASA.
The Far Frontiers Sector is one of the furthest flung of all of the official sectors of Traveller (only exceeded by Group One's Theta Borealis Sector). It lies a two sectors spinward of The Spinward Marches, just past Foreven. Half of the sector is dominated by the Zhodani Consulate, while the other half contains a number of pocket empires, some of which support the Zhodani and some of which support the Third Imperium.
All of the FASA adventures were set in the Far Frontiers sector, which is probably the prime reason to run a game there; many of FASA's thinking-man Traveller adventures are considered classics. Unfortunately, there was no official description of the sector as a whole until well after FASA stopped publishing. If you decide to run in Far Frontiers, you'll probably need to pick up Ares Magazine Special Edition #2 (which had a short article on part of the sector) or else Traveller Chronicle #2-8, which expanded that article, then went well beyond it to produce a very complete look at the area.
If you wanted to expand the Far Frontiers sector into a full domain-sized area of space, you could supplement it with the Foreven Sector to trailing (which I'll talk about next) as well as the Vanguard Reaches Sector to rimward and the Beyond Sector to trailing-rimward. The latter two sectors were land grants to Paranoia Press, but I haven't given them full descriptions in this article series because they were the foundations of their own sector books and not much else (as Paranoia Press spent most of their time putting out rule books and gaming accessories, not settings or adventures).
Foreven Sector . The Foreven Sector lies just spinward of The Spinward Marches, so it should be a great place for adventure, especially since it's even more of a melting pot of the Zhodani and the Imperium than the Spinward Marches, with several client states of each of these mighty empires lying scattered between them (much like the Far Frontiers sector). Unfortunately, it's received almost no support to date, and is now officially a "blank land", open for any GM's individual interpretation.
Personally, I hate blank lands, because as a GM you have the ability to replace anything that you want anyway. Naming Foreven a blank land just means that we'll never get an official look at an area that should be pretty important to many campaigns because of its proximity to the Marches. On the plus side, you can download an official outline of Foreven Sector online. In addition, some third-party Traveller publishers (namely, Hell Creek Sanitarium, Jon Brazer Enterprises, and K Studio) are just now starting to publish Foreven Sector supplements, under a special license from Far Futures.
At the moment Foreven is too sparse to run a campaign within, unless you do a lot of work on your own. However, I suspect that'll change within the next year.
Gateway Domain. The one area of the Traveller universe that's always been described as a domain is the Gateway Domain. It includes Crucis Margin Sector , Gateway Sector (aka Maranatha-Alkahest Sector), Glimmerdrift Reaches , and Ley Sector . The Gateway Domain is an interesting locale because it's a set of largely unaffiliated worlds and clusters that lies between the K'Kree's Two Thousand Worlds, the Hiver Federation, and the Third Imperium. This is the sort of true frontier that's been the heart of most of the Traveller campaign settings: just on the edge of the Imperium and interfacing with a couple of other societies.
Of the various sectors I've described in this article, the Gateway Domain is the only one that has had two largely incompatible descriptions.
The Gateway Domain was originally the home of the Judges Guild Traveller supplements (and thus the earliest land grant). Judges Guild published all four sectors as standalone books and put out numerous adventures set in them. For reasons that boggle me, GDW decided to throw out all of Judges Guild's work by revising all the star positions and names when they published the Atlas of the Imperium. Still, if you want a fully detailed domain supported by almost 20 modules, the materials describing the non-canon Gateway Domain are all still out there (though from what I hear, the adventures aren't that great on average).
In more recent years a new, canon version of the Gateway Domain has been published in two very well-received books. The first was MegaTraveller Journal #4, which features "Lords of Thunder", an extensive campaign by the Keith brothers, set in the Rebellion time period. The second was Gateway to Destiny, which was set in the Gatway era, when the nearby wars between the Solomani and the Third Imperium have their own effects upon this region. There are several PDF adventures and sourcebooks which have followed up on this latter book, making it a very rich location for adventure.
Besides the four sectors that make up the Gateway Domain, proper, there are also two nearby sectors that have received some attention. One is The Hinterworlds, which lies just rimward, and which I'll talk about separately, because it was released as a standalone sector (and in a different time period). The other is the Empty Quarter Sector , which has been the default sector for the online fanzine, Stellar Reaches; it's been presented largely as an add-on to the Gateway Domain and lies just coreward.
Though I'll talk about the big two Traveller campaign settings--The Solomani Rim and the Spinward Marches--in my next article, the Gateway Domain certainly has the potential to be the third, thanks to the fact that it's a real nexus for stories and that it's been detailed in good books by some of the top authors for the game. Unfortunately Mongoose GMs will have to extrapolate from these sourcebooks to come up with an 1105 Golden Age Gateway setting.
Hinterworlds . We've already looked at Diaspora, the second sector for MegaTraveller. GDW's first half-hearted attempt to detail a sector for MegaTraveller occurred when they detailed the Hinterworlds sector in Challenge #39.
It's a sector with as much potential as the classic frontier sectors of Traveller, because it's situated between the Third Imperium, the (somewhat distant) Two Thousand Worlds, and the Hiver Federations. Thus, as with the other true frontiers, you've got a few different alien races competing with the Imperium. Even better, the sector itself belongs to no one, but instead is the home for a bunch of pocket empires.
Unfortunately, the sector was never supported, except as a place to set Challenge Magazine articles. As such, it's probably largely forgotten and inaccessible to modern Traveller GMs.
Having finished my look at this first set of Traveller settings, let me offer one caveat: though I've constantly linked to topical tags in the RPGnet Game Index, they're not yet complete. Consider them starting points, but not comprehensive listings of all the publications for these settings.
Conclusion
In this first article I've covered several of the more detailed areas of the Traveller universe: the Core sector, the Diaspora sector, the Far Frontiers sector, the Foreven sector, the Gateway domain, and the Hinterworlds sector. Of them, I find the Core and Diaspora sectors to be of somewhat limited use because of their non-frontier nature and/or their more limited details.
However, the Gateway Domain is surely one of the great campaign settings for Traveller, the Far Frontiers is a setting with some great adventures, and the Foreven and Hinterworlds sectors could supplement other settings or stand on their own.
And I still haven't gotten to the two greatest sectors, the Solomani Rim and the Spinward Marches, both of which will appear in my next article.
