What these numbers are for
Every drill, game and lesson on this site records what you copied and what you got wrong. This page turns that into the three things worth knowing: which characters you are actually losing, what you mistake them for, and whether you are improving.
Which characters, not which feel hard
People are poor judges of their own weak characters. The ones that feel difficult are usually the ones you notice yourself hesitating over, which is not the same as the ones you get wrong — a character you miss cleanly and instantly leaves no impression at all. The heatmap here is measured rather than remembered, and it routinely disagrees with what people expect.
The pairs matter more than the characters
Most errors are not random. They are substitutions: you write an A where an N was sent, or a U for a D. Those pairs are usually each other reversed, and drilling the two of them together for five minutes is worth far more than another general practice session, because the fault is not that you do not know either character — it is that your ear has not separated them.
Where the page finds a pair you confuse, it offers to drill exactly those characters and nothing else.
Speed is a trend, not a number
Your speed on any single session means very little — it depends on the material, the time of day and whether you were paying attention. The line over weeks is the real measure, and the useful thing about it is spotting a plateau early. Flat for a fortnight usually means it is time to change something rather than do more of the same.
Stored here, and nowhere else
All of it lives in your own browser, in local storage. There is no account, no server and nothing uploaded — which also means it is not backed up and does not follow you to another device or survive clearing your browser data. That is the trade for not having to sign up for anything. See the privacy page for exactly what is kept.
Weak characters are best attacked directly: the character blitz drills a chosen set for sixty seconds, and the practice library has a deck of the pairs that most commonly trip people up.