Understanding Windmill Types: A Guide to What You're Looking At
Most people can identify a windmill at a distance — the sails are a reliable clue — but the differences between types are not obvious until someone explains what to look for. The distinctions matter for visiting: a post mill rotates its entire body to face the wind and is a very different experience inside from a tower mill where only the cap moves. Knowing the types also helps with the map, where mill types are recorded as tags and can be filtered by architecture.
Post mill: the oldest type
The post mill is the oldest surviving windmill form. The entire body of the mill — a wooden box-like structure called the buck (in England) or kast (in the Netherlands) — is mounted on a large central wooden post and can be rotated to face the wind. The miller rotates the mill by pushing on a long tailpole extending from the rear of the buck, using either their own weight or a horse and chain.
The engineering consequence is significant: everything in the mill — millstones, gearing, sack hoists, the miller's living quarters in larger examples — must fit into the rotating buck. This limits the size of a post mill. The largest English post mills are about ten metres long; Dutch ones can be slightly larger. The minimum internal floor area is cramped.
The oldest surviving operational post mill in England is the Outwood Mill in Surrey, dated to 1665. The Saxtead Green Post Mill in Suffolk, maintained by English Heritage, dates to at least the early eighteenth century in its present form. In the Netherlands, post mills were gradually superseded by tower mills from the seventeenth century, but many survive as monuments, particularly in North Holland and Friesland.
Smock mill: the octagonal specialist
The smock mill takes its English name from its resemblance to a traditional smock garment — an octagonal tapering body built of timber weatherboarding, usually painted white or tarred black. Only the cap (the uppermost rotating section) turns to face the wind; the body is fixed. This allows a substantially larger and more productive mill than a post mill, because the body can be as tall as the structural materials allow.
Smock mills are particularly associated with Kent and Sussex in England (the Cranbrook Union Mill is the finest surviving example, a six-storey smock mill built in 1814) and with the Dutch polder drainage tradition. The octagonal plan distributes the structural load efficiently and provides eight flats for the weatherboarding, making the smock mill both practical and architecturally distinctive.
The cap of a smock mill typically carries a fantail — a small auxiliary vane at the rear of the cap that rotates the cap automatically to keep the sails facing the wind. This invention, attributed to Edmund Lee in 1745, removed the labour-intensive requirement for the miller to manually orient the mill and was adopted rapidly across northern Europe.
Tower mill: brick and stone permanence
The tower mill replaced the post mill and smock mill as the dominant form wherever masons and bricklayers were available. It consists of a round or polygonal masonry tower — brick in lowland areas, stone in upland ones — with a rotating cap and sails at the top. The body is fixed; the cap turns by fantail or manual winding gear.
Tower mills could be built much taller than timber equivalents, allowing them to catch better winds above the surrounding landscape. They are the dominant surviving form across most of Europe: the stone tower mills of La Mancha, the brick tower mills of Lincolnshire and East Anglia, the rendered-masonry mills of the Netherlands and Denmark. Tower mills are also more durable than timber structures; a well-built masonry tower can survive indefinitely without maintenance, which is why many derelict windmills visible in the European countryside are tower-mill shells.
Hollow post mill: Dutch drainage variant
The hollow post mill (grondzeiler in Dutch) is a Dutch variant of the post mill developed specifically for drainage. Like a conventional post mill, the body rotates on a central post; but the post is hollow, allowing a rotating shaft to pass through it to drive an undershot scoop wheel or Archimedean screw at ground level, below the body of the mill.
This was a crucial innovation for Dutch polder drainage. A hollow post mill could pump water continuously while rotating to face the wind, without requiring any mechanical connection between the cap and the drainage machinery below. The type was in widespread use in the Netherlands from the sixteenth century through the nineteenth; surviving examples include several in the Zaanse Schans area and around the Kinderdijk drainage system.
Tjasker: the Frisian minimum
The tjasker is the simplest form of wind-powered drainage, used almost exclusively in the Frisian province of the Netherlands. It consists of a single diagonal shaft — a long wooden or metal beam — mounted at an angle so that one end carries two or three sails and the other end drives an Archimedean screw directly into the ditch or watercourse. There is no building, no tower, no rotating cap. The entire machine is the shaft plus sails plus screw.
Tjaskers were inexpensive to build and maintain and were the drainage solution for small landholders who could not afford a full post or hollow-post mill. Hundreds operated across Friesland in the nineteenth century. Around forty survive, several in working order and maintained by the Frisian mill society (De Fryske Mole). They are visually unlike any other mill type and often surprising to visitors who encounter them for the first time.
Function: grain, drainage, sawing, paper
Mill type and mill function do not map onto each other in a simple way. Tower mills and smock mills were used for all the major functions; post mills were mostly grain mills. The key functions were:
Grain milling (the most common): wheat, rye, barley, oats, malt. The majority of surviving windmills were grain mills at some point in their history.
Drainage: scoop wheels and Archimedean screws driven by wip and hollow post mills, removing water from polders. This is the function that defines Dutch windmill engineering and the Kinderdijk UNESCO site.
Sawing timber: wind-powered sawmills (paltrok mills in the Netherlands) used an eccentric crank to drive reciprocating saw blades. The Zaanse Schans preserves a working example. Wind-powered sawmills were instrumental in Dutch shipbuilding in the seventeenth century.
Paper-stamping: hammermills driven by wind crushed rags into pulp for paper manufacturing. The Zaan area had a significant paper-mill industry. De Schoolmeester at Westzaan is a surviving example.
What the map records
On the map, mill type is recorded where data is available from OpenStreetMap surveys and heritage databases. You can use the map to compare the distribution of post mills (heavily concentrated in England, Belgium, and Poland) with tower mills (dominant in the Mediterranean, Scandinavia, and the Netherlands) and drainage mills (concentrated in the Dutch and Flemish polderlands). The geographic pattern of types reflects both geology (stone available for tower building) and function (drainage needs concentrated in low-lying areas).
Knowing the type before you visit tells you what to expect inside — and whether the building you're looking at is a single-room rotating box or a five-storey masonry tower with four floors of machinery.