This article is based on traditional musicology as well as artistic research. On the one hand, I will try to cast a new light on the role of the composer and performer Élisabeth-Claude Jacquet de La Guerre (1665-1729). The research on her has until now mainly been focused on her life and works, not so much about her role as a performer. However, she had a vital role as a harpsichordist and organist in Paris and the court of King Louis XIV. The French gazette and literary magazine «Mercure galant» mentions several times her performing abilities and stated that she played and sang the most difficult pieces from sight and that she accompanied and composed on the spur of the moment. I will base my paper on this and other contemporary accounts. I will also include a discussion of the «Prélude non mesuré» – the non-measured prelude, which she uses in her first book of harpsichord suites. These preludes captivated listeners with their richness of dissonances and Italian-influenced timbres and were a strong indicator of her ability as a great improviser and performer on the harpsichord. For performers today, however, these preludes might sometimes be an enigma. The rhythmic flexibility of the pieces with their whole note notation maintains the sense of improvisation; however, this also refers to the main problem, since the notation is not always clear. To the modern performer, who is familiar to conventional notation, there seems to be very little rhythmic guidance in the score. However, by applying grouping of notes, stress, and duration – while at the same time noticing appropriate stylistic conventions – performers can form the prelude into a comprehensible whole: the difficulty lies in understanding the score.