Hi, it seems that most of the THREE.Geometry documentation are wrong.
It says for the PlaneBufferGeometry
Each of the contructor parameters is accessible as a property of the same name. Any modification of these properties after instantiation does not change the geometry.
But the properties are accessible through the property named properties
There is also a typo to the word constructor
Have a nice day 
@rroux-gpsw the documentation is created by the community, so if you see an error it would be great if you fixed it. There’s an ‘edit’ button in the top right of each page.
That said, in this case I think it’s just badly worded. As you say, it says:
Each of the contructor parameters is accessible as a property of the same name
…and yes, you can access each of them via PlaneBufferGeometry.parameters property. Also, changing them after instantiation does nothing, so that part is correct.
If you want to rewrite this more clearly, please do!
@looeee Hey, thanks for the reply!
I dind’t knew that the documentation is editable, I’ll change this next time.
Have a nice day!