In the healthy shoulder, a natural kinematics rhythm/timing exists between glenohumeral ABD and scapulothoracic lateral rotation. After 30 degrees of abduction this rhythm is 2:1, meaning that for each 3 degrees of abduction, 2 degrees occur in the glenohumeral joint and 1 degree occurs in the scapulothoracic joint.
This very nice video and article by Physio-Pedia makes it very clear.
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