It is preference or science that determines when you use a 16, 17, or 18 real seat size?

I saw a Falcon rod with a FUJI TPS 16 seat. None of my printed catalog had one just like that. Are they available to to general public?

Comfort of the angler and ability to fit the reel that is being used are the primary concerns when selecting reel seat size.

As an example, many rod builders just slide on whatever size seat is the closest fit to the blank. I had a customer who had had several ultra-light rods made, all of them had #16 reel seats on them. He found them uncomfortable for his very large hands. I built him an entire set of rods from ultra-light to heavy striper and all used the #20 Fuji seat. He was delighted and said they were the most comfortable rods he has ever fished. Also, he likes the fact that all his rods have a similar "feel in the hand" regardless of what size, length or power the particular rod might be.

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Agree with Kirkman. I have big hands (small brain). Little reel seats are uncomfortable and actually cramp my hands. Preference is the key word.