The address, in pieces: prefix, body, tail
What a v3 onion address is
A v3 onion address is 56 characters before the .onion suffix, drawn from a 32-character alphabet: letters a to z, digits 2 to 7. The last 14 characters are a checksum derived from the service’s key. That is why one wrong character rarely resolves at all: a mistyped string lands on a different site or nowhere.
The prefix part
The leading characters of an onion address come from a vanity search over keys. hn2paw7z appears at the front of all three strings on this site because the service keeps it across address changes. It is also why lookalikes paste so cleanly into search results: the first eight characters match, and the eye stops there. The prefix is the easy half to check.
The tail is the hard half
The eye pattern-matches the first eight characters and stops reading. The last 14, the checksum tail, are exactly where a wrong character hides. Copy the full string; never retype it.
How the eye fails
Tor Browser’s address bar is narrow, and narrow bars truncate long strings. The middle disappears into an ellipsis, right where clones like to hide characters. Paste the string into the page and use find-in-page (Ctrl+F or Cmd+F); a full match on the tail stops mattering once the bar can show it.
The paste tool
The tool at the checker compares a pasted string against the three strings on this site, character by character, and points at the first position where they differ. It runs entirely in your browser and uploads nothing. Unofficial and unaffiliated, like everything on this site.