Legato playing does not come naturally to most students.
For adults and teens (and older elementary students), usually all you need to do is mention the problem and perhaps have the student demonstrate correct technique. Sometimes I take a pencil and "connect the dots" of the notes which must be connected, just as a reminder, also pointing out that repeated notes are impossible to connect.
Little kids are another matter. They have real physical problems getting the fingers to work so that one comes up exactly when the other comes down.
I explain legato vs staccato as the difference between hot fudge sauce and M&Ms. (I am rather fond of chocolate and find most of my students are, too, so the analogy works well.)
I ask the student to demonstrate those two ideas to me. If the student can't do both (which I expect will be a problem), I show the student and ask if she can hear the difference. Many times the student has not fine-tuned the listening skills yet! When asked to attend to the difference, the student immediately can differentiate them.
We work together until we have at least a couple of notes strung together legato. The most likely fingers will be 1-2 or 2-3. If a student is having trouble with this movement, confine early attempts to strong fingers and a minimum number of them!
For the next week, I assign a "hot fudge exercise," in which the student just plays back and forth on those two fingers trying to get legato. Sometimes the student can do two sets of fingers. Sometimes at the lesson the student gains quasi-control over all four two-finger sets. In this case, the assignment for the week is "hot fudge twosies," with special attention to the pairs the student identifies as the most problematic. In some cases, only one hand is plenty challenge; other cases, the child can work twosies in both hands (but hands apart).
At the next lesson, we try threesies. Depending on this how goes, we'll do threesies at home the following week or maybe move on to foursies or even fivesies. Hands are still apart.
When fivesies, hands apart, is well established (maybe a month for a kindergartener!), we put hands together. At this point, I like to -start- with the problem combinations. I discuss it with the child, who sees right away that those guys need some extra work, so starting with them will allow them all to end up even in the end.
We work through twos, threes, fours, and fives, as described above.
Meanwhile, in the songs, I point out the need for hod fudge in the places where the student has demonstrated by the hot fudge exercise that he can play legato. I often take a brown pencil and "connect the dots" (chocolate, you see).
At the point, legato is not well-established in the literature, so I go back to an "old old baby song" and use it for a hot fudge song. This is a song that has been passed off for at least a month. We do a weekly hot fudge song until the student is firmly established in his pieces with legato playing as the "default."
After this regimen, occasionally students will need a reminder, but a verbal hit is usually enough. If this won't do the job, go back to the brown pencil!
copyright 1998, Martha Beth Lewis, Ph.D.
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