GeezerVison

This could use a list to make the difference between the two builds stand out more, and to mirror the two sections below. Something like:

Here you could also add link to :term:s like GC and GIL using intersphinx.

I added a section at the end to summarize the differences between the two implementations. I was hesitant to add it here because we introduce a number of the concepts later on in the document.

Do each --+ mark one bit? If so, ob_tid, ob_ref_shared and *ob_type are 2 bytes (16 bits), but this fields don't seem to match the description, since ob_gc_bits takes ~4 bits and ob_ref_local 6.

No -- each row is 64-bits (on 64-bit platforms), but otherwise the fields pad, ob_mutex, ob_gc_bits, and ob_ref_local are not to scale.

I'm not sure what to do about this. It'd be nice if the diagram were to scale, but I think that would require the diagram to be very wide -- ob_gc_bits is only 1 byte, but 10 characters.

It might be enough to just add a note after the diagram saying that e.g. "Note that pad, ob_mutex, ob_gc_bits, and ob_ref_local are not to scale and take X, Y, Z, W bytes respectively."

Another option, if those 4 values are related, could be to use a single name (like "free-threaded info bytes") and then break that down afterwards, either in text or with a separate "zoomed in" diagram.