Cutting sleeves on the bias.

This Threads article describes a great tip by Denise Severson, a seamstress, alterations expert, and Association of Sewing and Design Professionals member, to add sleeve fullness attractively: cut sleeves on the bias.
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Besides a more attractive fit, sleeves cut on the bias fit more comfortably and allow a wider range of movement because fabrics almost always have the most stretch in the bias direction.
Sleeves are not long and large pieces of fabric that can have a bit of weight to them, nor do they have have to support a lot of weight. As such, the extra stretch is not as problematic as is the case with a bias bodice or bias long skirt, where the size and weight of the fabric might result in some distortion in the garment’s shape.