The sagittal plane is the plane of a body 'slice' in a front to back direction.
The blue colour shows the sagittal plane.
Anatomic plane
If there was a very thin gap between two glass doors, and you turned sideways, holding out your arms out to the side up to shoulder height, to slip through the gap, then the glass doors would be considered to be in the sagittal plane in relation to your body.
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