Look at this painting.
It's Saint Francis and Pope Honorius III.
You can probably find the monks.
It's the hair.
This is not just a haircut.
The more you look, the more this haircut shows
deep religious divides.
One style was even lost to time, after being
banned by the Roman Catholic Church.
The scalp is a statement of faith, but it's
also a battleground.
Hair's religious rite extends far beyond
Christianity.
Some Buddhist monks shave their heads and
some Orthodox Jews don't shave the corners
of their heads.
The Catholic monks were known for centuries
for their particularly distinctive hairstyle.
This haircut, with the center shave, is called
a tonsure.
It started in the 4th or 5th century.
And the most recognizable is the Coronal tonsure,
possibly modeled on Jesus' crown of thorns
on the cross.
It's actually one of three types.
The Coronal is the Roman, or Petrine tonsure,
after Saint Peter.
There's also the Pauline tonsure, named after
Saint Paul, and used more commonly in Eastern
Orthodoxy.
It is a fully shaved head.
But in the Dark Ages, there was a third tonsure
too.
And that's the shape that largely disappeared
from the Church.
That hairstyle was a visible symbol of diverging
faiths and that's the reason that it was banned.
When Pope Gregory sent missionaries from Rome
to the British Isles in the late 6th century,
he found differences between the Roman Catholic
Church and Celtic Church.
Ones that revealed serious disagreements about religious practice.
Celtic Catholicism was out of sync with the
Roman Catholic Church.
Roman Catholics would later use the differences
between them to portray Celtic Catholicism
as Pagan or even as an offshoot celebrating
the power-hungry magician, Simon Magus.
There were concrete disputes.
Most importantly, they disagreed on when to
celebrate Easter and another significant disagreement
was the shape of the tonsure.
McCarthy wanted to learn the shape of this
tonsure, because it represented the split
in the Roman Celtic Churches.
He thought the old guesses about its design
were wrong.
You can't just scroll through photos of 7th
century monk haircuts.
Figuring out the shape of the tonsure these
monks use, is a detective story.
It required McCarthy to parse texts like the
Book of Kells and records of old letters.
From that, he could figure out 
the shape.
These old texts and illustrations only gave
McCarthy a view of the front and back of the head.
To picture an aerial view, he had to build
one.
These differences over tonsure were outward
signs of a split in the Church.
When the Roman Catholic Church took Ireland,
they slowly changed its tonsure too.
In 664, the king of Northumbria agreed on
the Roman Catholic date for Easter and the
Roman Catholic tonsure.
The change wasn't instant, but over time the
triangular tonsure disappeared.
Today some monks practice tonsure while others
don't.
It varies across religion and monastery.
In the Roman Catholic Church, clerical tonsure
ended in 1972.
When it was common, this unusual haircut was
a powerful symbol of monastic separation and
the Church's power.
But it's actually 
not so strange.
