Hi Alan
I wrote the same thing in two places Chigau words.
Because both write, and write your response here and there.
555 has an internal network with 100kohm
The window comparator. (18kohm Another theory says).
Power and network they are connected to GND,
2 / 3 of the "control" is terminal.
Meanwhile, the "trigger", "threshold" between networks and terminals 100kohm
Equivalent circuit will have no hysteresis.
Used to detect when the control terminal, the protective resistance 100kohm used.
The temporarily stable,
Network "3 / 3-2 / 3" Resistance and 100kohm
50kohm and parallel equivalent.
Therefore, the "threshold" terminal "3 / 3-2 / 3" is close to the resistance 51kohm used.
Instability because the show circuit simulator,
1nF capacitor connected to both.
Please try to prove too.
kyoji