Let's start with some pedantry. Yes. I know that ratios are a type of measurement. What I mean is that artists care more about the relationships between details more than they care about the unit measurements.
You might recognize this shape as the Golden Spiral, which is born from the Golden Ratio which, itself, is created from triangles adding up in ever-increasing size.
Take a close look in the middle. You'll see a small triangle with another triangle right next to it.
Then a third triangle with the same length side as the lengths of those two first triangles put together.
Then a fourth triangle is added with a single side equal in length to the first three triangles.
On the process goes.
The relationship of the new triangle to the ones that come before is the Golden Ratio (1.618).
Notice we didn't talk about what the size of the beginning triangle is. 1 inch? 1 foot? What's the measurement?
We don't care.
The interesting part of the ratio is the relationship between the triangles. This pattern exists within the ratio of the triangle separate from the lengths.
The pattern shows up regardless of the scale.
This spiral can show up in a tiny seashell, in a sunflower, or wood shaving.
No matter what the size of the medium, it's the ratio of the pattern that stays constant.
What Does This Have To Do With Artists?
You might have heard about the Golden Ratio showing up in art, architecture, sculpture, photography, etc because it's inherently beautiful.
Whether that is true or not is a different article.
The Golden Ratio means something to artists because artists are often dealing with ratios and the whole relationship of the parts to the whole more than they are concerned with a specific unit of measurement.
When you're making a portrait, the first two points you put on the canvas define the ratio of all the other points of interest in relationship to the distance you created with the first details.
This is a game of ratios that's defined at the very start of the process. #NoPressure
Ratios Gone Wild
It might help to look at obviously wrong examples to understand the "right" way of thinking about this.
The wrong way? Caricatures.
How in the world can you instantly recognize who its supposed to be?
The nose is way too bulbous. The forehead way too big. The eyes way too small.
It shouldn't look like anyone.
But it does.
It looks exactly like Bill Murray.
All the relationships between the details are correct, even if the details themselves don't make sense.
Artists are constantly playing with the space between details, and that's really where the magic of the likeness comes through.
This is why artists care less about what the detail "is" and more about how & where it fits in with all the others.