[6] According to Ernst Ludwig Gerber, Friederici built 50 organs in his lifetime, of which the one in Chemnitz is considered to be the most excellent.

[7] His craftsmanship shows the influence of Trost and Silbermann, while the disposition of stops reflected the practices of the galant era.

[9] Of the three surviving pyramid pianos, Stewart Pollens concluded that only the one in the Goethe House is likely by Friederici owing to similarities with the 1745 engraving and with Silbermann's pianoforte;[10] Michael Cole dismisses all three as inauthentic.

The name is either a French corruption of fortepiano, or a pun referring to hitting the instrument good and hard to get it to produce a sound.

[13][14] Daniel Gottlob Türk describes it as a square piano in 1789,[14] and Joachim Hess [de] reported that the instrument had eight stops in 1774.

[15] While authorities of the 19th century, such as Fétis and Alfred James Hipkins consider Friederici as the inventor of the square piano, following Heinrich Christoph Koch's claim that he invented it in around 1758,[16] the year may have been mistakenly added by Koch after seeing Friederici's name appear in Jakob Adlung's Anleitung published that year.

Both makers' clavichords had strings an octave higher in the bass register of the instrument, which Bach disliked.