Flag of the Free City of Frankfurt (1815-1866)
History
Frankfurt is named after a ford in the river Main that the Germanic tribe the Franks controlled. It was first mentioned in 794, when it was part of the Frankish Empire. From 855 the city was the place where German kings and later emperors of the Holy Roman Empire were elected (but until 1562 crowned in the imperial city of Aachen).
In 1372 Frankfurt received imperial immediacy, meaning that it was directly represented in the empire's institutions and not subject to some overlord. In 1792 the last emperor was crowned in Frankfurt, before the city was conquered by Napoleon in 1806 and made part of a larger Duchy of Frankfurt.
Frankfurt was located between the Hessian states, Nassau and the north western tip of Bavaria.
After Napoleon's defeat Frankfurt became a free city again, now in the German Confederation. It was a cradle for German nationalism and liberalism, and in 1848 it was the first place where the German black-red-gold flag was used. During that revolution an attempt was made to unify Germany in a constitutional (and democratic) monarchy but that failed after it was opposed by conservatives. It was a crucial part in history where Germany continued to have autocratic regimes as opposed to its western European neighbours.
In 1866 Prussia and Austria went to war over the control of Schleswig-Holstein, but effectively over which would be the most dominant state in Germany. Frankfurt sided with Austria and as a result it was annexed by Prussia.
Frankfurt's central location made it a trading hub through history. The city had a relatively large Jewish community, of which the Rothschild banking family and Anne Frank are famous examples. Modern Frankfurt, though 5th in size in Germany, is still the country's financial center today.
Currency
Brakteat (until 1372)
Comparable to other German mints thin Pfennig coins called Brakteaten were minted in Frankfurt as well. In the 14th century the Groschen was introduced at a value of 12 Pfennige.
Goldgulden (1386-1612)
Frankfurt was part of the Rhenish Monetary Union that was founded in 1386 based on the Goldgulden. The Goldgulden was an imitation of the Florentine Fiorino d'Oro and was divided in 20 Albus each of 12 Heller. By 1436 this had been adjusted to 24 Albus each of 9 Pfennig or 12 Heller.
Gulden (1612-1838)
From 1612 the Gulden was subdivided in 15 Batzen, 20 Kaisersgroschen, 30 Albus, 60 Kreuzer, 240 Pfennig or 480 Heller. The Reichsthaler was generally seen as worth 1½ Gulden and the Speziesthaler as 2 Gulden.
Frankfurt followed the general conventions on silver content in the Holy Roman Empire over time in relation to the Reichsthaler. Not long after the Conventionsthaler was introduced in 1754 it did follow other southern German states in devaluing from a 20 Gulden standard to a 24 Gulden standard in 1766.
South German Gulden (1838-1866)
The South German states adjusted their Gulden to a 24½ to the Cologne Mark standard in order to align it with the Prussian Thaler which was on a 14 Thaler standard. Their common coin was double denominated 2 Thaler and 3½ Gulden. From 1857 Frankfurt started minting Vereinsthalers which were worth 7/4 Gulden, but kept the Gulden as well.
Gold coinage
The Goldgulden increased in value versus silver money over time until it reached a level of 3 Gulden around 1700. A Ducat was estimated at 4 Gulden in the 18th century and 5½ Gulden by 1838.
Catalogue
Imperial mint (before 1372)
https://en.numista.com/catalogue/frankfurt_imperial_mint-1.html
Free City (1372-1866)
https://en.numista.com/catalogue/frankfurt_free_imperial_city-1.html
Note: I would suggest to merge above two issuers into one.
