This Tangent piano is one of the 3 known, being made by Johann Wilhelm Berner who was according to Ernst Ludwig Gerber's Historisch-biographisches Lexikon der Tonküstler (from the end of 18th century) active in Spath & Schmahl's workshop in Regensburg before settling in Hamburg.

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According to Gerber he was a very successful maker towards 1800.

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He followed the design of his colleagues Spath & Schmahl in all details except that he used black keys for the accidentals, used mahogany veneer similar to English pianos which were in fashion in Hamburg and Scandinavian countries and also made sometimes square tails in English, Flemish, French harpsichord style while Schmahl ones had round shape like Stein.

Another difference of Berner's tangent pianos to those of Schmahl is that he used double bridge (the brass section being divided from the long bridge which holds soft iron strings). This feature was not unfamiliar in German speaking countries. The famous Viennese piano maker Johann Schanz also made from the very early on, grand pianos (about 1803) with divided bridge just like Berner at the same time in the north but mainly the idea must have became popular from England (in the early 1790s Broadwood and Stodart started to make such bridges)  but even earlier in Germany there were clavichords made with divided bridge. So it is not necessariliy an English invention but we can assume that it was the late 18th century English grand piano which made Schanz around 1800 in the South and Berner in the north to adopt this idea on their grand pianos.

Schanz as a Viennese maker had to make pianos as viennese as possible though, while German makers felt free to adopt and adjust whatever idea they found useful from anywhere. Berner adopted more from England on his tangent pianos, because apart from the cut cheek and the lid's furniture style but also the music stand the general looking is more or less like an English grand (specially because of veneering and white natural keys, mahogany case. The nameboard is typical Swedish/Danish.

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The surviving tangent pianos are the following :

1- Private ownership (Germany) with typical Stein-Spath half round tail dated 1796

2- Radbon Fortepiano Collection (Germany) with square tail (probably 1797)

3- In the museum of music in Copenhagen, belonged to Christian VII the Danish king, dated 1798 (with square tail)

 

The tangents, action, dampers, instruction and many more features are the same as Schmahl which is no wonder of someone working there for decades making the same thing sending it to ''All corners of the world''! (according to other 18th century sources)

The weight of tangents, their shape, stringing gauges (therefore volume and solidity and sonority of the sound) were never changed from at least 1780 to 1801 (in the workshop of Spath and Schmahl) except that Schmahl made ca.20cm shorter version tangent pianos as well as the longer ones which were around 210cm) and used overspun strings in the bass (one for a pair, the other one plain).

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Tangent piano was a very popular instrument unlike what we would imagine today (being a special instrument).

The number of Tangent pianos by Schmahl alone is more than the whole surviving number of pianos by Walter and Stein (both being very well known at the time!)

Tangent pianos have light and rapid action despite having no escapement. Tangents are kind of ''Pivotless hammers'' hitting the strings like harpsichord jacks but with an intermediate lever between them and keys which makes the action much easier to play especially with a low key-depth (similar to Stein!) which can be directly compared with Erard's double pilot (which was originally a patent by the famous German maker in London Johannes Zumpe).

 

One of the reasons that the tangent action is very rapid is because the tangents are free and jump extremely fast, while this action's relatives (basically Stoss-mechanik) like those of Zumpe (Simple action) or Cristofori/Silbermann (with escapement and even back check and intermediate lever altogether) or English grand action, non can be compared to the tangent action with its fast reponse of the fingertips! it can only be compared to Stein pianos.
Appearantly German musicians of the second half of the 18th century appreciated only a very light and fast action and cared less for the little bit more volume of the typical English pianos.

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As a matter of fact even the simple Prell-mechanik which has also no escapement is faster in response of the fingertips than simple-stoss action (which was also in common in Germany but indeed less than Prell-actions.

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Stein's Prellmechanik has its own character and feeling for touch and all Stoss actions need to be played a little different, but the tangent action being a kind of Stoss action has still its advantages of being Stoss-mechanik but the same time having the fast response similar to the Prell-mechanik. 

 

The tangent action was indeed not an invention of Spath but from da Zwolle (15th century) and maybe even earlier. Indeed in its simpler form but still ''jacks hitting the strings and not plucking'' 

In the 18th century the main maker of the tangent piano was doubtless Spath who introduced this instrument and made many of it and sent it globally to even Russia.

Later his son in law Schmahl married his daughter and made this kind of piano (but also normal fortepianos and maybe square pianos etc. as usual). Spath died in 1786 and Schmahl continued up to at least 1802. His sons continued longer till at least 1814 but mostly made the usual fortepianos.

 

Tangent piano was not only made by Spath & Schmahl and their pupil Berner but other makers either made tangent pianos more or less similar to these three main German makers  (like Baldassare Pastori and some few unknown makers in Italy) but also many others who converted hatpsichords, spinets or pianos to tangent-action pianos.

A few examples exist even in England and Poland of originally made tangent action square pianos. In one case a Longman & Broderip grand but since it is prisioned in an isolated private ownership we can hardly have any information from this instrument. Most probably it was later converted to a tangent piano.

 

In Germany there were a few more makers (with less importance, compared to Spath & Schmahl and Berner) who made their own impression of tangent pianos. Mostly unsigned...

 

 

It is not absolutely clear if the ''Späthische Claviere'' Mozart referred to in his letter to his father in 1777 were Spath's normal fortepianos or his widely-known tangent pianos which were made at least as early as 1751 in his workshop.

But there is no doubt that tangent pianos were known to Mozart and that he played on them.

The same is about Haydn, cpe Bach and other contemporaries.

We don't know ''officially'' if they loved this kind of fortepiano or not but we should be aware that there were many of them made in these countries and had many buyers, so it is easy to accept that they must have been very favorite amongst musicians.

We can also imagine that someone like Beethoven may have found them less serious than Stein and Walter in his youth and no doubt for a period he used to play Streicher, Graf and such bigger and louder pianos. So, performing the music around 1750 up to 1810 on tangent pianos is historically and stylistically very right. 

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More detailed information will be updated here but reading more about tangent pianos (articles by Michael Latcham and Giovanni di Stefano) is highly recommended.