Today, musical instruments are generally tuned in the so-called equal temperament. This tuning system has the advantage that you can play equally well in any key. However, in the equal temperament the intervals are not just, but slightly out of tune, which leads to audible beats. In particular, the third differs uncomfortably strong from the just third.
If you want to achieve a better purity of the sound, you will quickly reach the limits with static tuning systems, because pure intervals are only pure in certain keys for mathematical reasons.
The free app presented here is intended to demonstrate that it is possible to implement an adaptive dynamical tuning scheme which produces just intervals in each key. The app analyzes the played chord, calculates the optimal frequencies and instantly tunes the built-in sampler or the connected MIDI device in real time. How such a music sounds, you can find out here: