Five basic patterns are suggested here. These will be used in later guild design. They aren't exhaustive, and some clearly shade into others. But these patterns are perhaps the commonest, and any DM can develop them to suit his campaign. One factor which isn't considered here, but which will be looked at in guild design, is how many solo operators, non-guild members, are around. This will affect the strength of the guild, but won't affect its style of organization so much.
How a guild is actually led will be dealt with straight after this section, but for now we'll assume that all guilds are led/ruled by a guildmaster. This won't always be so, but using this convention saves a lot of tediously repeated qualifiers.
Centralist[]
This is a strongly organized guild, where all guild members have a powerful primary loyalty to the guildmaster. Loyalty is mostly high, the guildmaster is usually a powerful and dominant figure, and the guild house is a major meeting place. Thieves from such a guild will work with a notable degree of co-ordination.
Cohesive[]
Again, there is a fairly strong central organization and authority. The guild is a unified body and its rules are readily accepted by all. However, there is room for some "local latitude." This may take the form of senior thieves below the guildmaster being able to make a few important decisions about affairs on their own patch (own quarter of the city, etc.), or the central rulership deliberately restricting the use of its authority in some cases, and so on.
Fractionated[]
In this case, there is a central thieves' guild still. However, individual figures within the guild have taken power unto themselves in important ways. Local thief leaders may insist on almost complete discretion to do what they want in their areas, while agreeing not to intrude on other local leaders' territory. Power struggles within the guild will be commonplace under these circumstances.
Oppositional[]
The guild has been burst asunder by tensions within it, and there may well be several competing guilds in the same town or city. This can happen because the guildmaster is weak, because of ambitious young Turks seceding from the guild, or for special reasons of many sorts (e.g., racial enmity between a guild of dwarven thieves, and one of elven and half-elven thieves, in the same town).
Relations between the different sub-guilds can vary considerably. If there is still the shell of a central guild, then the different guilds may be on fair terms still. However, at another extreme, the tensions and oppositions can degenerate into street warfare and gang assaults.
If there are, in effect, several guilds, then the DM will have to determine the organization and leadership of each one separately.
Anarchic[]
There is no guild organization to speak of, although there may be the remnants of a once-strong guild or perhaps a young, slowly developing guild. However, the large majority of thieves work as unregulated individuals. This may lead to a dog-eat-dog outlook with lots of backstabbing, both metaphorical and literal. However, it's equally possible that thieves get by with individual one-to-one agreements, helping each other out as and when the need arises (this may be especially likely in a Chaotic Good society with an anarchic guild, for example). Anarchy doesn't preclude enlightened self-interest and cooperation; it just tends to preclude this happening on a systematic and organized basis.
Complex/Mix[]
As usual, this covers a range of options. There may be a facade of one organization, whereas behind the scenes a quite different organization presides. It may seem that anarchy rules among the thieves of one town, but many are secretly organized behind the scenes. Or, a guild might seem to be centralist whereas in fact local leaders are hiding much of their activity from others, so that the guild is in reality fractionated. A complex situation could exist whereby the guild looks centralist at first sight, then looks fractionated (as above), and then is seen to be part of each (a couple of the strongest local leaders are secretly allied to carve up more territory while other leaders are fractionated)—a really complex option. Ample scope for DMs to develop intrigues and onion-skin puzzles here!