Demi-filées are metal-wound strings on a natural gut or silk core, with the characteristic that the winding is not tight, but spaced, so that the string is essentially wound in half, hence the name in French. In reality, in the examples of demi-filée that have come down to us, the winding is much more spaced.

The sound is intermediate between that of a bare gut string and a wound one, and is therefore particularly suitable for the middle register of the Baroque violin (D), Viola da gamba family and Three-order harp, as a timbre transition string. Or it is also historically attested for the basses of the Lute, archlute,  or theorbo.

The demi-filée is essential for the performance of Forquerais' repertoire, which requires extreme agility in the lower part of the Baroque bass keyboard, where the C generally responds very slowly, which is why we have developed the Set Forquerais, which includes two demi-filées (C and G).

The invention of the demi-filée, however, could have derived, as Mimmo Peruffo claims, not from a timbre requirement, but from the inability to draw the metal beyond a certain limit, which made it impossible to make a tightly wound string, unless very thin cores were used, which were therefore extremely fragile (in addition to the fact that the timbre would become strongly metallic).

For example, the D of the demi-filée violin was used only by French violinists in the 19th century, but this is explained by the fact that the Parisian pitch was very low, and therefore the D in bare gut would have been extremely thick and therefore dull. For the standard baroque pitch of 415, however, a demi-filée is not at all necessary, and our Cordoncini di Roma are not only sufficient, but actually has a much richer and more powerful sound. 

After years of research, we have finally rediscovered how the historic demi-filées were made! They no longer have the defects found previously, that is, the metal wire no longer accumulates near the bridge or the nut during tuning, and the bow hair is no longer blocked by the metal coils. The new demi-filées combine the timbre of gut with the agility of wound strings, in a perfect compromise.