The organ was built somewhere between about 1440 and 1457 by Harmannus of Groningen, an organ builder from just across what is now the Dutch border. The later date is the one that appears in writing: the chronicler Eggerik Beninga recorded 1457, and for want of anything firmer that is the year usually given. In 1513 the instrument was worked on again and a small balcony was added for the organist to sit on.

What makes Rysum remarkable is not that a fifteenth-century organ exists - cases and carvings survive in a number of places. It is that the pipes survive. Rysum is generally described as the oldest instrument of its kind in northern Europe that still has most of its original pipework, and it is named alongside the organs at Sion in Switzerland, St Valentin in Kiedrich and Ostönnen in Westphalia as one of the oldest playable church organs anywhere.

The specification is short enough to read out in a breath: Praestant at eight foot, Gedackt at eight, two Octaves at four and two, a Sesquialtera of two ranks, a Mixtur of three to four, and a Trompete at eight. Everything not made by the twentieth-century restorers is gothic.

But it did not begin as an instrument you could register. It began as a Blockwerk, in which every rank sounded together whenever a key went down - one enormous, unvarying, unstoppable sound, which is what medieval church music wanted from an organ. By the end of the seventeenth century, and possibly as early as that 1513 rebuild, someone had fitted a slider chest so that individual ranks could be selected. That change was not cosmetic. Congregations had started singing, and an organ that could only roar was no use for accompanying them. The proof is a payment record: a builder named Kayser was paid for seven turned stop-knobs, which means that by his day there were seven things to turn on and off.

The organ is tuned in meantone, slightly above a1 = 440 hertz. This is the tuning that Europe used before it agreed to compromise, and the compromise it refuses to make is the interesting part. In meantone, the major thirds in the common keys are pure - genuinely pure, without the faint beating that a modern piano's thirds all carry. The price is that a handful of remote keys become unusable, howling badly enough that organists simply avoid them.

The result is that music written for this instrument sounds, in it, the way it was designed to sound. Wind comes from wedge bellows at seventy millimetres of water pressure, gentle by later standards and part of why the tone is what it is. Recordings by players such as Harald Vogel have carried it well beyond East Frisia.

Between 1848 and 1910 an organ-building firm held the maintenance contract, and the nineteenth century did to Rysum roughly what it did to a great many old instruments. In 1867 and 1868 the case was cut down to make room for a new ceiling in the church. Around 1880 to 1890 the keyboard and stop-knobs were replaced with modern ones. None of this was vandalism; it was ordinary competent updating, of the kind that has quietly destroyed most of the medieval organs that once existed.

The repair came in 1959 and 1960, when Jürgen Ahrend and Gerhard Brunzema took the instrument apart and put it back the way it had been. They restored the gothic facade, remade three ranks of pipes from hammered lead by the old method, and returned the whole thing to meantone. It is one of the founding jobs of the modern historical-organ movement, and it is why the instrument sounds like itself again.

Rysum was first mentioned in the tenth century and is built on a warft - one of the artificial dwelling mounds people in this coastal country raised, basket by basket, to keep their houses above storm tides. The village holds roughly 689 people across ten square kilometres of the Krummhörn. The Reformed church at the top of the mound dates from the fifteenth century, which is to say it is about the same age as the thing inside it.

East Frisia has an improbable concentration of historic organs for a region this small and this flat, enough that the phrase Orgellandschaft Ostfriesland - the organ landscape of East Frisia - is a real category rather than a tourist-board invention. Rysum is its oldest voice.