Climbing difficulty is a spectrum from the easiset thing worth climbing to the most difficult thing anyone will ever climb

A grading scales communicates where on a this spectrum a particular climb falls.

Here's the first portion of the French scale

This is the Yosemite Decimal System which is common in the United States.

Let's focus on a smaller segment of the scale, to start addressing some detail

At this level, it's a bit more accurate to visualize grades as a range of difficulty. Climbs are (kind of) points along this spectrum.

There's a climb in Yosemite listed for each grade. And two listed that fall within the 5.10a (Crest Jewel and Sons of Yesterday).

From here we'll be focusing on what ranges these grades communicate.

In just the area around 5.10a to 5.10d there's (much) room for interpretaion.

Climbers regularly use the grade 5.10 without an a, b, c, or d.

Is it a middle-of-the-road grade, right in the center of the 10s?

Or is it a more general grade, that vaguely refers to anything within the range?

This issue compounds when considering the + and - variations on every number grade.

All 10s could be ordered one after another in a single series

Now 5.10 looks very broad when used as a general grade for all other 10s.

Or, do the pluses and minuses refine the broad stretches of whole-number grades

It would also make sense if they represent just the upper and lower halves of their respective numbers

But… in practice, it's not uncommon to climb at an area that consideres a + to mean harder than the number implies. or a - to mean probably easier than the number implies

This situation complicates again when climbers use a slash between whole numbers, 5.9/10 and 5.10/11.

And complicate again when using a slash for letters 5.10c/d.

Similar issues also affect other grading scales, like the French system mentioned above.

Translation between different scales is at least as messy as the issues above.