The fourth north transept window

Here we see a flagellum and a pair of keys. The flagellum was a form of whip used for scourging. It was made of leather with leather thongs and had pieces of leather or metal knotted into the thongs so as to tear the flesh.
The Jewish practice was to deliver thirty-nine lashes but the Romans had no limit – the victim was scourged until almost dead –
“Five times I [St. Paul] received at the hands of the Jews the forty lashes less one”. [2 Cor 11:24].

There is one silver key and one gold key for which there are two interpretations. One interpretation is that the silver key symbolises the power to bind and loose on Earth, and the gold key the power to bind and loose in Heaven. The other interpretation is that the silver key represents “binding” and the golden key represents “loosing”.

The crossed keys are the “keys to the Kingdom of Heaven” and are the quintessential symbol of the Papacy as an institution. Indeed, the crossed keys are part of the papal insignia shown above –
“And I tell you, you are Peter, and on this rock I will build my church, and the gates of hell shall not prevail against it. I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven.” [Matthew 16:18,19].