Sizing Key Mortises (was inconsistent rail pins)

[Ric Brekne]

Hi folks

I always liked the balloon analogy.  Take a balloon and and blow it half 
way up. Then with a tush pen draw a bunch of equal sized circles all 
over its surface.  When you then further expand the balloon what happens 
to the circles (holes) ?  And what happens when you let the air out a 
bit (contraction). 

This analogy came up a few years back when we were scatching our heads 
about comparing summer/winter tuning pin tightnesses.

Cheers
RicB