When transcribing music for bowed instruments to the guitar, bowing indications can give us valuable insight into how the music should be articulated. However, they can also mislead us if we assume they are invariably included for musical purposes. Sometimes they clarify technical aspects of bowing, which for us guitarists means that we should not read them necessarily as guitar articulations. Let me explain with the following example from the Fugue of Violin Sonata BWV 1001 by J.S.Bach.
J.S.Bach: Violin Sonata BWV 1001, Fugue, mm. 29-32
Bow indications are scarce in this Fugue (and others), for the polyphonic texture requires a specific, mostly unambiguous way of bowing, and the sixteenth-note lines of the episodes are to be played detached. A different case is, for example, the Adagio, where chains of ornamental single notes can be articulated in many different ways. When bowings are included in the Fugue, it is likely because performers would be musically inclined to group the notes differently.
Such is the case in this example. These bowings are technical in nature, to prevent the passage from sounding “choppy” by changing bow directions at every eighth note. They are included because they are musically counter-intuitive: the single eighth notes anticipate the next chord, instead of deriving from the previous one. Many guitarists interpret them as “appoggiaturas,” playing strong chords that “resolve” to the single eighth notes, which come slurred and softer. (Many violin players also follow this interpretation, aiming to achieve a “sigh” effect, which some treatises considered as actually implied in the slur markings.) We should notice that the single notes are anticipations instead of resolutions. They belong to the next chord’s harmony instead of resolving a dissonance from the previous chord. Then, grouped in pairs of single eighths/chords, it becomes clear that these are iterations of the repeated notes of the Fugue’s theme.
It is interesting to notice that the slurred, “sigh” interpretation does agree with the dynamic intent, for the single anticipating eighths should be softer than the chords, just in the same way that the repeated notes of the theme should be played crescendo. In the guitar, however, we can also be coherent with the detached articulation of the theme simply by not slurring throughout this passage. Always question the markings on a score!