Hold a magnet to a bar of iron and the tiny domains inside it turn to face the same way. Take the magnet away and most of them turn back. Not all of them. What is left over, the magnetism the iron keeps after the field that made it has gone, is called remanence.

It is an old word from the Latin remanere, to remain, and it has quietly held up a great deal of the modern world. The first reliable computer memory, in the 1950s, was a woven plane of ferrite rings the size of grains of rice. Each ring held one bit by remanence alone. You could cut the power, and when it came back the machine still knew what it knew.

The mark

Our mark is a hysteresis loop, the curve you get when you drive a field up and down through a piece of iron. Follow it round and you will notice it never passes through the middle. At zero field it sits above the line. That point, the dot on our mark, is the remanence.

Why it is the right name for what we build

Most software today is magnetised by somebody else's field. Your assistant is only as good as the model you rent this month. Your history lives in an account you do not control. When the field moves, the iron falls back to nothing.

We want to build the other kind of thing: the parts that keep their shape when the field is gone. A partner whose memory and rules are a file on your own disk, that is still itself when you change the model underneath it. That is troth, and it is the first of them.