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Writing/Legibility/
Slashed Zero
When designing letters for an alphabet, or numbers for a numbering system, it is important to make sure that individual characters are clearly distinguishable, even under adverse conditions. It should be easy to tell letters apart even in poor lighting condition, from an angle, when the character is moving, if parts of the character are cut off/not visible (Though of course there are limits to this), viewed from a distance and, ultimately, quickly. The goal of good letter and number design is to avoid a situation in which a reader needs to double check that they got a character right, even if they only caught a glimpse of it. Rule 1: Individual letters and individual numbers should be very clearly distinguishable. Characters should have clear, defining features that set them apart from other characters.
Example: a, e, o.
All of these share commonalities, but have serious differences that you can easily spot if you're prepared for it. a less so, depending on how you actually write it, but even if you write it like an o with a funny stroke on the bottom right and a straight line up on the right side, that's still good enough to distinguish it.
Now, everywhere around us there are strings in which both characters from the alphabet and numbers can appear. This text is one such string. License plates are such strings. Any one of the plastic cards in your wallet will have multiple such strings, especially your bank card. Any email will contain such strings. Passwords. In fact, most strings you see throughout the day can probably contain both letters and numbers next to each other. This means that the number zero and the letter o can appear in the same string. The problem between an uppercase O and the number zero is that they have no distinguishing feature. Yes, the O is usually wider than the zero, but that's only usually and depends on the font or the style of someone's writing. Without a distinguishing feature, you as the reader have to hope that the writer is consistent with their styling of zero'es and O's. Without a distinguishing feature, you as the reader will have to go back and cross-reference any O-shaped characters and verify that they indeed are what the context implies they are.
And sure, it's possible to work it out. It's possible to find another character to cross-reference, to estimate character widths, to work it out based on context, but let me tell you what: Recognizing characters should not be a fucking riddle. A reader should never be forced to compare an unknown character in the text against one they already know; at this point your lettering system has failed. And even then, a reference character might be far away, making it hard to compare. Or a reference character might not exist at all, forcing you to guess based on your intuition of the width of the character you're looking at. Every character should be individually recognizable without context, without needing to cross-reference it.