Classifying Neighborhoods and Properties A to Z - A discussion

[James Masotti]

I saw a post from @Joel Owens on this thread and it made me wonder how everyone identifies their property types. 

By this I mean...you'll regularly hear investors say stay away from D class neighborhoods. Or I only invest in B+ neighborhoods or better. 

Now I imagine this is some what subjective and is possibly even different from single family to multi family or from urban to suburban but the questions I put out on the table is:

What scale do you use to classify neighborhoods and properties?

And 

What is your criteria for each step in the scale?

Discuss