Your eye jumps eight letters, and skips your typo

You reread the message five times before hitting send, and the typo is on the second line. There is a physical explanation. When you read, your eye does not glide along the line: it jumps seven or eight letters at a time and only sees sharply what falls dead centre, about four letters either side. The rest is filled in by your brain with what it expects to find — and in your own text, what it expects to find is what you meant to write.

Print shops knew this before laboratories did. In 1631 the royal printers Robert Barker and Martin Lucas put out an edition of the Bible that had lost a "not" from the seventh commandment: a thousand copies instructing readers to commit adultery. It cost them a fine of three hundred pounds — twenty years of a labourer's wages — and their licence to print. Hence the rule of the trade: the person who corrects is not the person who wrote.

You are not going to hire a proofreader for a three-line email, but you can do the one thing all their tricks do, which is break the familiarity. Before sending anything that matters, switch the text to a different typeface and read it from the last sentence to the first. The sentences stop making sense, your eye is left with the letters, and that is where the mistake is.