The Excluded Characters of Base58

The defining feature of Base58 is what it leaves out. By carefully pruning the standard alphanumeric character set, Bitcoin eliminates the most common sources of human transcription error.

1. Visual Ambiguity (The "lookalikes")

In many fonts, certain characters are indistinguishable. To prevent users from sending money to the wrong address because they misread a printed key, Satoshi removed:

• 0 (Number Zero)

• O (Capital O)

• I (Capital I)

• l (Lowercase L)

If you see a character that looks like a vertical line in a Bitcoin address, you can be 100% sure it is a 1, because I and l don't exist. If you see a circle, it is a 9 or an 8, because 0 and O don't exist.

2. Non-Alphanumeric Symbols

Base64 uses + and /. In many contexts, these symbols cause issues:

• Line Breaks: Software might think / is a folder path and break the string.

• Encodings: Some systems might convert + to a space. Base58 uses ONLY letters and numbers, ensuring it is "Safe" for every computer system on Earth.

3. The Alphabet

The resulting 58-character alphabet is: 123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz

Notice the missing 0, O, I, and l.

4. The "Check" Logic

While the alphabet itself reduces mistakes, it is the combination of Base58 + Checksum that makes it a "Financial Grade" encoding.

• The alphabet prevents you from misreading the string.

• The checksum prevents you from mistyping the string.

Character Pair	Problem	Result in Base58
0 vs O	Both look like circles	Both removed
I vs l vs 1	All look like lines	Only 1 remains
+ and /	Break URLs/Paths	Both removed

In the next section, we will analyze the Base58 Arithmetic used to convert numbers.