Bitcoin addresses in the older alphabet

Money

The older Bitcoin address form: an unbroken mixed case run that begins with 1 or 3 and carries its own checksum inside the tail.

The shape

Prefix: a single 1 or a single 3, and no other opening character
Total width: roughly 26 to 35 characters, counting the prefix
Alphabet: base58: the ASCII letters and digits with four characters removed, described at the alphabets
Case: mixed, and the case of each letter is meaningful
Separators: none at all; one continuous run with no spaces, hyphens or breaks
Checksum: derived from the rest and folded into the tail, never printed as a separate field

1EXAMPLEEXAMPLEEXAMPLExxxxxxxxxxxx   34 characters
3EXAMPLEEXAMPLEEXAMPLExxxxxxxxxxxx   34 characters

Placeholders. Each spells EXAMPLE three times and then runs twelve x characters. Neither would satisfy a checksum, which is the point of writing them this way.

Width does not separate the two openings. A string beginning 1 and a string beginning 3 sit in the same band and use the same alphabet, so the opening character is the only visible difference between them, and it is not a difference a reader can interpret from the string alone.

The four characters base58 leaves out are exactly the four that people confuse when reading a string aloud or copying it off a screen. They are named once on the alphabets page and deliberately not repeated on every card that uses them. What matters here is the consequence: if you see any of those four in a string presented as an address of this form, the string is not in the alphabet, and no amount of squinting will make it so.

In the alphabet

1EXAMPLEEXAMPLEEXAMPLExxxxxxxxxxxx

Not in the alphabet

1EXAMPLE0EXAMPLEOEXAMPLExxxxxxxx

The digit zero and the capital O are both absent from base58. One of them appearing means the string was retyped, read out of an image, or passed through something that decided to correct it.

How to check it by eye

The four absent characters

Read the opening character. It is 1 or 3. Anything else and you are looking at a different form or a different chain.
Sweep once for a zero and a capital O, then sweep again for a capital I and a lower case l. Four characters, two passes, a couple of seconds.
Count in blocks of ten using the method on counting by eye. You want a total in the high twenties or low thirties, not fifty and not ninety five.
Confirm the case is genuinely mixed. An all lower case string opening bc1 is the newer form, and it obeys different rules.

What a wrong shape means

A total width well outside the band is the commonest wrong shape, and it usually means a selection stopped early or ran on. Ninety five characters is a different chain entirely. Sixty four hexadecimal characters is not an address at all; it is the shape of a transaction id or a file hash.

One of the four excluded characters means human handling somewhere in the chain of custody. Somebody typed it from a screen, or read it out of a photograph, or an autocorrect substituted a character it considered more likely. This is worth knowing because it tells you the string did not arrive by copy, and a string that did not arrive by copy is a string that could have been altered without anyone intending it.

A wrong shape here is a statement about how the string reached you. It is not a discovery about anyone. Assuming otherwise leads people to look for an adversary while the real fault sits in a paste that lost its last three characters.

What the folded checksum can and cannot do

The tail of the string is computed from everything in front of it. Change any character and the computation no longer agrees, so the string is refused rather than accepted as some other address. That is a genuinely useful property, and it is why a slip of one key almost never produces a working address pointing somewhere unintended.

The limit of the property is easy to state. The check asks whether the string agrees with itself. A different address, produced properly by somebody else, agrees with itself perfectly. If a whole string was swapped for a whole string at any point before it reached your screen, every arithmetic test still passes, because there is nothing arithmetically wrong. Detecting that requires comparing the string against a record you kept yourself, which is a different activity from reading a shape.

What a correct shape does not prove

Suppose the prefix is right, the alphabet is clean, the width is in the band and the tail agrees. The following remain entirely open.

Who controls the key behind the address. Nothing in the characters carries an owner, a name or a role.
That this is the address you were given earlier. Only your own record can answer that, and comparing the two is a separate step from reading the shape.
That it was not exchanged for a different valid address between its author and your screen. Substitution preserves validity by definition.
That the chain is the one you assume. Prefix conventions belong to one system, but strings of similar width and alphabet exist elsewhere.
That the address is still in use, or was ever in use. A well formed address that has never appeared anywhere looks exactly like one that has.

What this card is not

The newer address form, with its single case rule and longer checksum, is on its own card, and the alphabet legend lives on the alphabets rather than being reprinted here. Addresses on the other chain covered by this reference start at a Monero address. If your address arrived inside a square pattern of dots rather than as text, read what a payment QR code carries first.

A Bitcoin address in the older alphabet