Good question. The strict legal answer would probably be: "Not very"
That being said, we have made 10s of thousands of boat numbers and never heard of a customer getting a ticket for fancy boat numbers. So, don't overthink this.

Why? Let's get into the weeds:

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Most States mention they want block letters. But, how do you define 'block'?

1. Some might not consider "Times Roman" or "Copper" block because of the flairs on each letter/number's 'legs'. But really these are the most 'blockish' letters out there.
2. Original Latin based characters were considered 'block' when they were capital letters. Lower case letters were considered cursive. By that strict definition, fonts we call 'the most cursive', could be defined block if you choose capital letters (ALL boat numbers are made with capital letters)
3. Let's be reasonable.
   It's possible to order very simple block letters at 3 inches and STILL be unreadable. Especially if you chose a similar second color (shadow / outline) and/or chose a similar color to your boat.

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So, whatever font you chose, be reasonable. Make sure its readable and you'll have no problems if/when the Officier inspects your boat. On the other hand, give him a hard time and get in his face, and well, you deserve a ticket for whatever he can find. Be nice and enjoy your boat. Have fun!

If you really want to make sure you are 'legal' and want to know the 'how and where' to place your new Boat Registration Validation Sticker... Click your State and find out the details.