See the three planes
Imagine a sheet of glass passing through the body. A movement belongs to that sheet when it travels along the glass instead of leaving it.
Connect a plane to a demonstrated movement, rather than to the page orientation.
There are three sheets, and each one has its own kind of movement. The sagittal sheet stands on edge down the middle of you, and movement along it goes forward and back: folding forward, an arm swinging up in front, a knee bending.
The frontal sheet stands across you like a wall at your back, and movement along it goes sideways: side bending, an arm lifting out to the side, feet stepping wide. The transverse sheet lies flat through you like a tabletop, and movement along it is turning: a twist, the head turning, a palm rotating.
Each one also has an axis, the line the movement turns around, like the pin in a hinge. Naming the plane is a way to describe a movement, not an instruction to split the body, and most poses use more than one plane at different joints.
Three sheets, three movement paths
Each view looks straight at the plane it describes.
This is a simplified learning diagram. It is not a reviewed anatomical illustration.
Standing Forward Fold
Choose one plane. Play the small demonstration, pause it, then reveal its axis.
Lift one arm forward, then out to the side, then turn your palm up and down.
Notice: Three different planes, three different words, one arm.
This is noticing, not instruction. Move as little as you like, stop at any point, and watching instead of moving is a complete way to take part.
Which plane best describes the demonstrated side bend?
Pause before opening Practice. Say the distinction in your own words.
Look at the moving region. What stays relatively still?
Never classify an entire complex pose as exclusively one plane. Midsagittal remains a small reference note rather than a focus.