A few millimeters goes a long way in pen design.

I learned that after introducing my "Mercury" pen model in October '22 at the Dallas Pen Show. I had visited my first pen show in just March of that year, in Baltimore where I was introduced to the Vintage pen market. I found that there were thousands of pens available in that one show which were produced in the 1920's through 1950's that all had a similar size to them. These pens from the golden-age of fountain pens all measured between 4-1/2" and 5" in length with a barrel thickness of about 7/16" or ~12mm.
This is a real practical size pen, especially for keeping in your pocket or purse, or for slipping into the loop of a notebook. 

But in the Custom Fountain Pen community, no one was making and selling this size pen. So I decided to change that........

Mercury pens next to a standard BIC® pen and a Sharpie® Marker. As you can see, my Mercury is actually about .5mm slimmer than a Sharpie, but unique in the "Maker" community for slimness.

After introduction of this new "slim" model at the Dallas Pen Show, I expanded the line of Mercury pens to additionally offer a longer one, which at ~135mm (5-1/4") is close in length to my mid-sized "IKE" and my larger size "Liberty" as show in the 1st photo.

So the relevant dimensions for these 3 pens are:

• Liberty:
• Cap diameter ~17.5mm
• Capped length ~140mm long
• 15mm cap to barrel threads
• Section pinch ~11.8mm
• Barrel diameter ~15.4mm
• IKE:
• Cap diameter ~16mm
• Capped length ~ 135mm long
• 13mm Cap to barrel threads
• Section pinch ~10.8mm
• Barrel diameter ~13.5mm
• Mercury:
• Cap diameter ~14mm
• Capped length
• "Long" ~135mm
•  "Short" ~122mm
• 11mm Cap to barrel threads
• Section pinch ~9.2mm
• Barrel diameter ~11.5mm

Size Matters