There are numerous ways for one to claim legitimacy. In most cases, legitimacy is established in multiple ways, which can include (but is not limited to) the following:

• appealing to popularity
• appealing to the strength of their military might
• appealing to their economic prowess
• appealing to culture
• appealing to history/past historical figures
• appealing to nationalism
• appealing to the party platform/agenda (e.g. "I seek to stamp out corruption")
• appealing to existing structures in society (e.g. religion, intellectuals, landowners)

Of course, by appealing to these "authorities", you're also granting them legitimacy. This can get rather complicated. A very simple example: when I exchange a sum of money from my debit card to the cashier at the grocery store, I'm appealing to authority of the banking institution that issued my card to show that I am able to pay for my groceries. This legitimizes the banking institution, because I am granting them authority over my transaction. In turn, the banking institution legitimizes me as my transaction is approved and I walk out of the store with a backpack full of food and cleaning supplies.

Basically, legitimacy is not a one-way street.

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In order to exemplify how people in power call on multiple sources to legitimize their authority, I'd like to discuss the celebrations for the 60th anniversary of the founding of the PRC on 01 October 2009.

Before I start, I would like to stress the following:

1. The following discussion should be understood as furthering an academic discussion on legitimacy itself, rather than a discussion on whether the PRC is, in fact, a legitimate ruler over modern-day China.
2. While I understand it's a violation of the twenty-year-rule (like, seriously, no shit), the paper I'm going to discuss talks about this event in an academic way, and I am going to refrain from discussing whether I personally buy into the PRC's legitimacy thing.
3. I'm mainly talking about it because it's within my field of study (that being the two major events in Maoist China, and, less specifically, modern Chinese history).

The paper:

Yih-Jye Hwang and Florian Schneider, "Performance, Meaning, and Ideology in the Making of Legitimacy: The Celebrations of the People's Republic of China's Sixty-Year Anniversary", China Review, Vol. 11, No. 1 (Spring 2011), pp. 27-55, accessed on JSTOR 09 July 2015 [link]

The 60th anniversary of the founding of the PRC is a major event for the Communist Party of China (alternatively the Chinese Communist Party, or the CCP). One can see why: by harking back to its historical origins, the CCP establishes its own legitimacy. After all, they've been in rule for 60 years at this point, thus, are they not legitimate?

Because the CCP cannot simply legitimize its rule on simply following "Leninist-Marxist ideology" as it did in the past (after all, the country, under Deng Xiaopeng and others, has sought to emphasize modernity while pushing the economy towards a more capitalist model), the CCP must find alternate means to establish its own legitimacy. This manifests itself in the following ways:

• Hu Jintao wears a Mao suit during the celebrations, while everyone else wears Western suits and ties (appealing to historic capital with the Mao suit, while appealing to the future with the Western suits)
• part of the celebration features Hu checking on the People's Liberation Army (PLA), as well as a showcase of the country's military arsenal (appealing to militaristic might; additionally, since the CCP historically has relied upon the PLA to stay in power, this segment serves to appeal to members of the PLA in order to ensure their loyalty to the state)
• the military arsenal is "made in China" (appealing to nationalism)
• the civilian parade starts off with four floats, featuring the following leaders: Mao Zedong, Deng Xiaopeng, Jiang Zemen, and Hu Jintao. Mao's float took five minutes, while the other leaders take four minutes. What is important here is not just what is said (that the current leader has links to previous exalted leaders -- appeal to history), but what is implied and not said (glossing over the Great Leap Forward and the Cultural Revolution, the latter which the party has been seeking to disavow since the 1980s; making the focus not on the Maoist era with these "disasters" but rather on events from the past thirty years; erasing Hua Guofeng from the Party's retelling of history, although he ruled from 1976-80).
• floats about the "brilliant achievement" of the CCP since the 1980s (appeal to successful policies, appeal to economic success, appeal to modernization and advancement)
• floats about how China is a unified country (appeal to popularity, especially with the depiction of the disabled and of ethnic minorities within China, as well as the float about inter-Chinese harmony with people of Chinese descent living abroad).
• floats depicting every single province/region of China (appeal to nationalism, appeal to unity). These floats include a float for Macao, Hong Kong, and Taiwan, emphasizing China's claims to these regions (two Special Administrative Regions and Taiwan, which asserts its own independence and its own government as the Republic of China).
• the use of children in many segments (if not all segments) (appeal to the future; children are often used due to their youth, their innocence, their cuteness, and their potential).
• the fact that this parade is entirely televised, with citizens of Beijing banned from being outside in certain areas and from attending (this allows the CCP to control the meaning and the message of the parade; the seamless camera transitions show off how effective the CCP is about arranging massive events, further legitimizing the CCP).
• the release of film The Founding of a Republic (appeal to history, appeal to culture with the large number of stars); the film itself puts Chang Kai-Shek in a more positive light compared to past portrayals, which further legitimizes the CCP's actions post-Mao.

You kind of get the idea. Through this one event, the CCP is able to appeal to multiple venues in order to establish its legitimacy. From the above, the CCP sought legitimacy by the following:

• appeal to history/to the past
• appeal to the future
• appeal to successful policies
• appeal to unity
• appeal to nationalism
• appeal to militaristic might
• appeal to economic success
• appeal to modernization/advancement
• appeal to culture

(There is probably more than this, but I'm trying to not bore you all.)

Furthermore, legitimacy is effectively a discourse (both vocal and visual), which the person in power seeks to shape in order to derive benefit from it. This is best exemplified by the CCP's appeal to history, in which many major events that are considered disastrous to the CCP are glossed over if not erased altogether, while events/ideas that further the current program of economic growth/capitalistic reform are given focus and positive discourse. By both appealing and controlling the discussion of the historical past, the CCP can use it to legitimize its power. This is not limited to the CCP; other rulers do the same thing.

The paper itself is an interesting read, and I highly recommend it if you want to get into the nitty-gritty details.

In conclusion, mods, please don't ban me or remove my post for outright violating the letter of the twenty-year-rule, as I did my best to follow its spirit. :P

I suppose the best go I can make of this particular question is to look at it in the context of the Habsburg Monarchy. The old and long-held view of the Habsburg's Monarchy from the Napoleonic Wars on has been one of decline. The downward trajectory from military superpower to frail, fragmented polity is one that has informed almost every work of Habsburg history since the fall of the Empire. The reasoning goes that as the notion of the ethnic nation-state gained more and more credence in the late nineteenth century, the Habsburg Monarchy and its dynastic control over an agglomeration of disparate nationalities were becoming more and more outdated. As each nationality grew bolder in its quest for independence, the nationalist leaders saw the Habsburg Dynasty as ever less important and legitimate.

New scholarship has shown that the story is not so simple and an entire new field of Habsburg revisionist historians have been trying to remove the sense of trajectory, of ultimate and inescapable decline, from the story of the Habsburg Monarchy. Laurence Cole and Daniel Unowsky, in their book The Limits of Loyalty make a rather valiant and convincing argument that the relationship between the Monarchy and the nationality groups is more nuanced that previously believed.

Habsburg claims for legitimacy in ruling a large and undoubtedly cumbersome collection of peoples rested for a long time on political tradition. "The Habsburgs have held the crown of insert province here for centuries, of course we're the legitimate rulers." But as the nationality question grew more pressing, the Habsburgs did indeed seek to establish their legitimacy as something beyond mere historical happenstance. They sought to incorporate national mythologies and histories into their own, showing that since time innumerable, the supranational Habsburgs had had a mutual relationship with their various national groups. In essence, the Habsburgs, considered a stale, stuffy, non-dynamic dynasty, were recognizing a new, emerging source of political legitimacy and tried to utilize it. Two of the essays focus on attempts by

the Habsburg authorities [to respond] to the growing influence of nationalist thinking by integrating selected national myths and local traditions into primary school instruction. The combination of dynastic tales and local/national figures may be uneasy, but apparently the Austrian state was promoting a compound loyalty in which the attachment to national-cultural homelands coexisted with and reinforced the allegiance to the emperor and the larger common fatherland he represented. In the case of Gynasium history instruction, however, the Austrian central state wanted to make sure that the main orientation in educating the future elite was an “Austrian patriotism” (p. 29).

The attempts were not always succesful, however.

The two essays by Nancy Wingfield and Hugh LeCaine Agnew show how supposedly “centripetal” dynastic symbols lost their integrating and uniting power in Bohemia. Wingfield’s deft tracing of the image of Joseph II is an excellent example of how an absolutist, pre-national reformist figure could be mobilized by different political forces.

Despite the failure, however, we can see a centuries-old dynasty trying to recreate itself by co-opting the national movements of the various peoples of the Empire.

Ultimately, however, in the Habsburg case, I am wont to define legitimacy in a more concrete sense. The legitimacy of the Habsburg realm among the majority of the Empire's peoples fall into one of two categories.

• 1. The ability to give smaller peoples the ability to act on the world stage.
• 2. Protection against a majority (in the cases of minority peoples).

The Austrians and Hungarians (Magyars) were both obviously the recipients of the benefits of controlling "imperial" lands. The increased manpower gave them population wells to draw on and several different breadbaskets to feed the empire. The Czechs even, a notoriously nationalist group in the late 19th century, only ever really pushed for increased autonomy. The Czechs realized that within the Habsburg system, that of a Great Power, they were able to ensure their national security, rights, and economic success with far more surety than if they were a tiny sovereign nation in Central Europe.

The second group contains many national groups, including the Ruthenians, the Croats, the Romanians and the Slovenes. The Ruthenians were especially likely to support the Habsburg Monarch, who they, as discussed by Unowsky, hoped might help them in their struggle against the dominant Galician Polish-speaking land-owning class. And yet even these same wealthy Poles sought to utilize the Emperor's symbolism in showing themselves to be highly loyal and stabilizing elements in Galicia.

To summarize, legitimacy can definitely be gained and lost. Legitimacy is a dynamic factor (at least in the Habsburg Monarchy) where it might, at a given time, be based on popular support, reformism, military strength (revolutions of 1848), support vis-a-vis ethnic majorities, or the ability to give smaller peoples the benefits of a Great Power.

Sourcs: Laurence Cole, Daniel Unowsky, eds. The Limits of Loyalty: Imperial Symbolism, Popular Allegiances, and State Patriotism in the Late Habsburg Monarchy. Austrian and Habsburg Studies. New York: Berghahn Books, 2007.

Ke-chin Hsia. "Bringing the Dynasty Back In." Book review of above title. March 2010. http://www.h-net.org/reviews/showrev.php?id=25294

(quoted passages come from Hsia's review of Cole and Unowsky's book)

Might as well get the ball rolling; for centuries, legitimacy in Europe relied heavily on Papal authority. There was a lot of continuity between the Western Roman Empire and medieval society, even if it was unrecognizable by the end. The Pope had a direct institutional connection to the glory days of the Roman Empire, so they carried immense prestige off of that, to say nothing of the power and prestige from being the head of the universal Church. Popes could release a prince's subjects from their duty to obey his laws, and his vassals from their oaths. This is an extremely serious threat in a society dependent on reciprocal relationships. In addition