1) Articulate legato. Since it is a piece written in the Baroque period, the basic articulation is articulate legato. The original term for this type of articulation used in the 17th and 18th centuries was the Ordinary Touch.
2) Small spaces. This touch means that you should leave small spaces between each note unless indicated otherwise by the composer. Since there are no special articulation indications in the original score, the notes should be detached.
3) Cantabile manner. Try to achieve the singing or the cantabile manner of playing. This term was used by Bach himself. For the best results try to play some passages from this piece using one finger only but as connected as possible. Then try to re-create the same articulation using the normal fingering.
4) Not too detached. Do not make the music sound too choppy. The best articulation will be if the listeners can't perceive the articulation but all the notes are clearly audible.
5) Strong beats. Try to emphasize the strong beats in each measure. This is possible to achieve in one of the 3 ways: 1) by making the weak beats a little shorter, 2) by making the downbeats longer, and 3) by coming in later on some more important downbeats, such as in cadences.
6) Inner voices. Articulate the inner voices in the fugue. While it is relatively easy to achieve the desired articulation for the outer voices (soprano and bass), the middle voices (alto and tenor) require your special attention. Therefore, it is very useful to practice the inner voices alone and in combinations with other voices.
7) Acoustics. The correct articulation also depends on the acoustics of the space. For example, if you are practicing at home, the spaces between the notes should be much less audible than in a vast acoustics of the cathedral or a church. The articulation in the concert hall should be somewhere in between the other two extremes. As you can see, the organist has to be prepared to adjust the articulation to the acoustics.
Use these tips in your practice of this piece today. If you are precise in executing every detail, you will be surprised how natural and stylistically correct your organ playing will become.
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