Horowitz and Michelangeli pianos.

In the first half of the 20th century, most pianists practically had a customized instrument made; I don't remember who had a piano with narrower keys due to having small hands... So it wasn't such an unusual thing. When Michelangeli took a piano, it was completely inspected, and if even a single felt pad was placed backwards, he could feel it. As for how he touched the instrument, Horowitz certainly used a lighter keyboard, and Gould's also had some peculiarities that escape me now, just to give a few examples; furthermore, the further back you go in time, the more different the instruments were from today's, making comparison difficult.

Yes, but the question is something else. Their pianos were not born that way, but were "transformed." However, I see a lot of talk about preserving weights, measurements, and hammers in their original condition. How does this concept fit together?

In fact, legends are sometimes created around these matters. Certainly, Horowitz's piano was prepared very specially by Franz Mohr, but some values cannot be highly customized. For example, one cannot make the downweights too light at the expense of the return weights. Perhaps one can intervene in a particular way on the regulation and tuning. Michelangeli played on many pianos... even on a Petrof grand piano, in the Vatican at the Nervi Hall. It is true, he was demanding, as every pianist should be. All those people who were not up to the task of assisting him made him out to be a maniac. He knew the functioning of the instrument very well. There is an audio recording of an interview with Tallone, who was the Maestro's assistant during a good period of his life, and it is very clear what kind of knowledge he possessed. In a recent video, where Michelangeli plays Brahms' ballads, one can extraordinarily notice up close how natural the position of the hand and fingers was. He "relies" heavily on the instrument, instead of "challenging" it, as the then Steinway Import Director Stefan Vertes told me during a lunch at my house in the '80s. He, Vertes, often maintained and said during presentations that the instrument had to be such, in terms of design quality and setup, so that the pianist could rely on it, and in this video the concept is obvious, visible. In conclusion, I think the Greats wanted and want simply to feel comfortable playing and expressing themselves without having to employ useless tricks and artifices. And I believe that a pianist's technical growth should aim for this. This is what I think.

So it's not true that they were more lightened? For example, my dealer told me that one can "apparently" play on the sensation of keyboard weight precisely with the tuning (a more open piano will seem easier and lighter).

Yes, it is true that a more open piano will seem more available, but only apparently. Everything also depends on the regulation and especially on the return weight. One cannot achieve an excessively light let-off weight at the expense of the return weight.

After playing on it for two hours, you then realize...

exactly