Tonality
Paganini intended the Concerto to be heard in E-flat major: the orchestral parts were written in E-flat, and the solo part was written in D major with instructions for the violin to be tuned a semitone high (a technique known as scordatura), so that it would therefore sound in E-flat. This enables the soloist to achieve effects in E-flat which would not be possible with a normal D tuning (for example, the opening of the third movement, where the violin plays a rapid downward scale A-G-F♯-E-D, both bowed and pizzicato. This is possible on an open D-string, but is not possible in the key of E-flat), because two strings would be required. The key of E flat would mute the sound of the orchestra, whose strings would play fewer tones on open strings, and this would make the solo part emerge more clearly from the orchestral accompaniment.
Contemporary audiences did not realise that Paganini had retuned his instrument, and were thus all the more amazed at what he appeared able to play. (The more musical members of the audience would have recognised the distinctive sound of a violin's open string, and would have observed that this fell on the keynote of work (E-flat), and would have therefore realised that Paganini had re-tuned his violin.)
Read more about this topic: Violin Concerto No. 1 (Paganini)