Arches
The arch of the foot is such an interesting feature. Some are high, some are low, some are extra-thin, some are very muscular, and most are a combination of factors.
I remember when my sister was in college. We had just been switched from the Army Medical Center to a private practice by our insurance. She was having pain in her foot and went to see the doctor. He took one look at her foot and told the visiting intern that she had collapsed arches. She was completely confused and assured the doctor that she didn't have collapsed arches and that the pain must be something else. (Side note: collapsed arches can happen from a number of problems, but she wasn't pregnant, didn't have flat feet genetically, and didn't think that the pain was great enough for a torn tendon). The doctor argued with her for a bit before letting her go seek a second opinion.
The podiatrist looked at her feet and complimented her on her beautiful arches and wanted to know how she had gotten them so strong and muscular. The pain in question turned out to be turf toe and plantar fasciitis, not fallen arches. Sheesh!
Last weekend, I did a total goof. I measured the dancer's foot on the Brannock (love that thing) and remembered to check the width. I forgot to check the arch length. I've only been selling shoes for 14 years. Give me time... I'll remember eventually.
The reason that checking the arch length is so important is that having a long arch (which the Brannock can measure) or a very high arch or a flat foot (which isn't as obvious) will change how the shoe fits. So this dancer was an American girls' size 2.5. The rule of thumb is two numbers smaller from American women's sizes and one number smaller from men's and children's sizes to get your Irish size. But, because of things like arch length, width, and toe shape, this is only a guideline. Not a set in stone rule.
So, I start out with an Irish 1.5. She can barely get it on and it clearly hurts. So I go up to the 2. That still isn't quite her size. I go up to the 2.5 and then she thinks that she can break them in. And her toes were still at the end (although, it's harder to tell with brand-new hard shoes than soft shoes). I couldn't figure it out. Fortunately, I hadn't adjusted the Brannock and my sister pointed out that this dancer, while her US length was a 2.3, her arches were a full size larger.
And to think that when I graduated college 14 years ago, I thought this would be an easy job!