Thirlage was the law in regard of the milling of grain for personal or other uses. Vassals in a feudal barony were thirled to their local mill owned by the feudal superior. People so thirled were called suckeners and were obliged to pay for use of the mill and help maintain it.
The term Thirl originated from the feudal past when a thirl was a body servant, retainer or vassal to a noble or chief. The term is interchangeable with 'Carl' and indicates subservience to the feudal lord and feudal laws; the situation being not that far removed from the conditions of slavery.
The obligations of thirlage eventually ceased to apply, but thirlage in Scotland was only formally and totally abolished on 28 November (Martinmas) 2004 by the Abolition of Feudal Tenure etc. (Scotland) Act 2000.
Similar requirements existed and were enforced actively in England.
Thirlage was the feudal law by which the laird (lord) could force all those vassals living on his lands to bring their grain to his mill to be ground. The law ensured that all the grain the vassals produced could be measured and thus taxed. Vassals had to carry out repairs on the mill, maintaining the lade and weir as well as conveying new millstones to the site. Such trees as beech and particularly hornbeam were grown as a crop to provide the necessary wood for the mill machinery.
The Thirlage Law was repealed in 1779 and after this many mills fell out of use as competition and unsubsidised running costs took their toll. This explain why so many mills ceased to be marked on maps after this date. Lambroch Mill for example was on the Annick Water near Stewarton in Ayrshire and apart from the weir and some other indications, it has entirely vanished, most likely because it was the thirlage mill for Lambroughton and its business went to Lainshaw or Cunninghamhead mill after the thirlage act was repealed.
Read more about Thirlage: Sucken, Multure, Grassum, Dishonesty, Quern-stones, The 1799 Thirlage Act, Transporting Millstones, Aid To The Poor, See Also