One Key to rule them all,
One Key to find them,
One key to bring them back and in the Darkness bind them
In the land of AH.Com where the shadows lie.

Originally devised by Thande, the idea is to give every country a colour so that maps showing far-flung colonial empires etc. can immediately be recognised. Other mapmakers have since added considerably and improved the UCS, but it has encountered controversy over whether it is overused in the Map Threads.

Slightly different versions of the UCS are used by different mapmakers due to difference of opinion, but the One Key, made by Calcion and euio, is now accepted by most mapmakers.

Colours are usually chosen based on historical data where possible. The original base colour scheme came from the Atlas of World War II.

There remains a debate as to whether there should be one UCS for ancient times and another, reusing colours, for periods after about 500 AD, but this seems to have been solved by the division of the UCS into four sub-schemes (Ancient, Medieval, Early Modern, Late Modern). Knowing the UCS fanatics around here, though, this will most likely be regarded as traitorous blasphemy. This has been accepted into the One Key.

Britain and British Empire Pale red - the British Empire was always red on contemporary maps, red being the colour of British uniforms, but the map ink tended to pale to pinkish. After the rise of the USSR, this pale red was used as blood-red had connotations of Communism.

France Blue, as French uniforms were blue.

United States Teal - the USA should also be blue, technically, but as France already has it, the US gets a greener shade.

Germany Grey, as in field-grey uniforms.

Imperial and republican Russia Gold to evoke the Tsarist flag. (Some people think it should be green to continue the uniform theme).

Soviet Union Blood-red for obvious reasons.

Italy Brown, taken from the aforementioned Atlas of WW2.

The current Universal Four-Period Version, compiled by Xeno:

The most advanced single-period version, by Calcion:

An earlier example of the UCS:

While the UCS is the largest unified colour scheme on AH.com, hence the name, of course only a relative minority of AH maps employ it (although it has become the dominant standard for OTL maps, as was originally intended). Some parts of it enjoy use outside the UCS as a whole, like certain colours for certain nations (blue for France and pale red for the UK, for example), but just as often even those standard colours are not used.

Scarecrow has let it be known that he is the most virulent objectionist to the concept of the UCS in and of itself, a fact stemming from an argument about Decades of Darkness maps with Thande.

There is one point in the UCS that does incite more moderate debate now and then: The issue of outlining vs colouring. In the UCS, colouring means de facto, and outlining de juro control, though some people disagree with that. The alternative mode of outlining/colouring has been dubbed Susanoism, due to Susano being its most outspoken proponent, but to his own surprise.