question: why are eggs almost universally sold in packs of 12? What made farmers agree to sell this way?
comment: TLDR; Convenience, 1 shilling = 12 pennies, 1 egg = 1 penny. No need to make change. Under a system that came to be known as English units, which was a combination of old Anglo-Saxon and Roman systems of measurement, eggs were sold by the dozen. It made sense to sell them that way because one egg could be sold for a penny or 12 for a shilling, which was equal to 12 pennies. That system held sway in the American colonies and persisted after the revolution, becoming part of the system known as U.S. customary units. Such units are used for consumer products and in industrial manufacturing. The British have moved on, adopting a wholly new system of weights and measurements in 1824. But they still mostly sell eggs by the dozen. Thus, in the United States, a vast majority of eggs are sold by the dozen, half-dozen and other multiples of 12. But in India and parts of Africa, it isn’t unusual to buy eggs by the piece, and in some countries they may be sold by 10s or 8s.
passages: An egg mark may contain a number of information parts - in the EU only the producer code is required. Additional information may be printed along with the date of production being the most common to find. Other information may contain the method of production especially in the non-EU world where the numbered levels do not apply.
   Chicken egg sizes
  
  Chicken eggs are graded by size, for the purpose of sales. The egg shell constitutes 8–9% of the weight of the egg (calculated from data in Table 2, F. H. Harms)
  
  Section::::United States of America.
  
  The United States Department of Agriculture sizing is based by weight per dozen. The most common U.S. size of chicken egg is 'Large' and is the egg size commonly referred to for recipes.
  
  The following egg masses including shell have been calculated on the basis of the USDA sizing:
  
  Section::::Canada.
  
  In Canada, modern egg sizes are defined as follows:
  
  Section::::European Union.
   Chicken eggs are graded by size for the purpose of sales. Some maxi eggs may have double-yolks and some farms separate out double-yolk eggs for special sale.
  
  Section::::Farming issues.:Color of eggshell.
   The central started with quality sorting and marking of all eggs in 1934 and the brand "Sol-egg" was introduces. During and after World War II there was rationing of eggs in Norway, but this was suspended in 1949. In the 1950s the egg central started active marketing through advertisements and in 1977 the "Sol-egg" brand was replaced with the Prior brand on all eggs and white meat.
   Note that egg labelling is used worldwide but mostly in the form of egg carton labels. In most countries there are legal definitions on the designation of the egg size, production method, packager identification and best-before dates. As there is more space on the carton there has been no tendency to create an egg code that is seen on the stamps used for egg marking.
  
  Section::::Producer code.
  
  The European Union has defined an egg code that consists of
  
  BULLET::::- a number indicating the method of production
  
  BULLET::::- a two letter code for the country of origin
   Section::::Producer code.:Registration number.
  
  In most countries the registration number starts with a region code where the egg production facilities are situated:
  
  BULLET::::- Germany - 01 Schleswig-Holstein, 02 Hamburg, 03 Lower Saxony, 04 Bremen, 05 North Rhine-Westphalia, 06 Hesse, 07 Rhineland-Palatinate, 08 Baden-Württemberg, 09 Bavaria, 11 Berlin, 12 Brandenburg, 13 Mecklenburg-West Pomerania, 14 Saxony, 15 Saxony-Anhalt, 16 Thuringia
  
  BULLET::::- Austria - 1 Burgenland, 2 Carinthia, 3 Lower Austria, 4 Upper Austria, 5 Salzburg, 6 Styria, 7 Tyrol, 8 Vorarlberg, 9 Vienna
  
  Note that a producer with multiple barns will have to register each barn separately.
  
  Section::::Criticism.
   Breaking and freezing of eggs began in U.S. in the 1890s in Minneapolis, Minnesota. George Dole, who owned one of the first cold storage facilities in the United States, and former school teacher H. J. Keith were the first to open an egg-breaking business, the H.J. Keith Company. They also began separating yolks from whites and selling them as separate products. In 1898, Keith made a deal with H. A. Perry of Salina, Kansas. Perry was the founder of Seymour Foods, and had invented a hand-operated yolk-and-white separator which dramatically improved productivity. Keith licensed this technology from Perry and dramatically increased production.
   In Europe, modern egg sizes are defined as follows.
  
  Section::::Post-Soviet countries.
  
  In countries which are members of Interstate Council for Standardization, Metrology and Certification: Russia, Belarus, Moldova, Kazakhstan, Azerbaijan, Armenia, Kyrgyzstan, Uzbekistan, Tajikistan, Georgia, and Turkmenistan eggs are sorted into five categories by mass:
  
  Section::::Australia.
  
  In Australia, the Australian Egg Corporation defines the following sizes in its labeling guide.
  
  In Western Australia, two additional sizes are also standardized by the Golden Eggs Corporation
  
  Section::::New Zealand.
  
  In New Zealand sizes are based on the minimum mass per egg:
  
  Section::::Brazil.
  
  In Brazil sizes are based on the mass. :
   From the farmer's point of view, eggs used to be practically the same as currency, with general stores buying eggs for a stated price per dozen. Egg production peaks in the early spring, when farm expenses are high and income is low. On many farms, the flock was the most important source of income, though this was often not appreciated by the farmers, since the money arrived in many small payments. Eggs were a farm operation where even small children could make a valuable contribution.
   quantity compared with the yolk. On the whole this egg may be pronounced not particularly palatable, and will never be used except as a makeshift, when neither hen nor duck eggs can be procured." A recipe for "Eggs an Miroir" instructs a reader to "spread a piece of butter upon a dish that can be set on the fire, break the eggs over it, adding salt, pepper, and two spoonfuls of milk; place it on a slow fire, with a red-hot shovel over it, and serve when the eggs are set."
  
  Section::::Content.:Selected chapter information.:Fish chapters.
   Unlike many products, trademarks and advertisements for egg brands are usually printed on the food container itself rather than on a separate container (as with breakfast cereals). This single-layer, distinctive packaging distinguishes egg cartons from different producers or quality on the retail shelf.
  
  Section::::Sizes.
  
  Standard egg cartons have room for 12 eggs, but they can come in a variety of sizes, holding from three to 24 eggs.
   The Egg Marketing Board was an agricultural marketing organization set up by the British government in December 1956 to stabilise the market for eggs due to a widespread collapse in sales. The Board purchased all the eggs produced in the UK, graded them to a national standard, and then marketed them to shops. Each eggshell was stamped with a small "lion" logo as a mark of quality that would be seen by the customer. Egg producers were paid according to the quality of their eggs, less a deduction based on the number of eggs sent to packhouses for administration and advertising. 
   Section::::History.
  
  American egg farmers approved the creation of the American Egg Board in a 1975 referendum, when they realized the need for one national voice promoting the Incredible Edible Egg. It was created by the Egg Research and Promotion Order pursuant to the Egg Research and Consumer Information Act of 1974. A referendum was conducted November 3–28, 1975, by the Agricultural Marketing Service and seventy-three percent of eligible producers approved the program. The Egg Board established by the order became the American Egg Board beginning July 9, 1976.
   Chickens and other egg-laying creatures are kept widely throughout the world and mass production of chicken eggs is a global industry. In 2009, an estimated 62.1 million metric tons of eggs were produced worldwide from a total laying flock of approximately 6.4 billion hens. There are issues of regional variation in demand and expectation, as well as current debates concerning methods of mass production. In 2012, the European Union banned battery husbandry of chickens.
  
  Section::::History.
   In medieval Britain there was an egg throwing festival held in the churches at Easter. The priest would give out one hard-boiled egg which was tossed around the nave of the church and the choirboy who was holding the egg when the clock struck twelve would get to keep it.
  
  Dutch children play a game called "egg sales" in which one child sells an egg to another. The new owner then throws the egg in the grass and if it does not break it must be returned to the seller.
   For the month of January 2019, the United States produced 9.41 billion eggs, with 8.2 billion for table consumption and 1.2 billion for raising chicks. Americans are projected to each consume 279 eggs in 2019, the highest since 1973, but less than the 405 eggs eaten per person in 1945.
   The 2013 FDA Food Code states that in serving highly susceptible populations (preschool age children; older adults; individuals with compromised immune systems; and individuals who receive meals through custodial care-giving environments such as child or adult day care centers, kidney dialysis centers, hospitals, or nursing homes ): “Pasteurized eggs or egg products shall be substituted for raw eggs in the preparation of Foods such as Caesar salad, hollandaise or Béarnaise sauce, mayonnaise, meringue, eggnog, ice cream, egg-fortified beverages and recipes in which more than one egg is broken and the eggs are combined.”
   After Eights were originally made from dairy-free dark chocolate. However, in 2002 Nestlé started adding butterfat to After Eights made at certain production facilities, so as to increase resistance to chocolate bloom. This practice expanded to all production facilities in 2009. Nestlé has also made special editions of After Eights, including orange After Eights and milk chocolate After Eights.
  
  Section::::Manufacturing.
   The company head office in the US is based in San Francisco with farm operations in various preferred US locations. Since 2012 the US brand has grown to supply over 4,000 retail outlets across the country including Wal-Mart, Kroger and Safeway stores and has since launched its free-range eggs in 6 count cartons into the market.
  
  Section::::Marketing.
  
  Section::::Marketing.:Nice Pecks.
  
  Inspired by an academic study showing that hens are calmer in the company of cockerels, but knowing that cockerels mixed with chickens result in fertilized eggs, the company created a pin-up calendar for hens in December 2012, 2013, and 2014.
  
  Section::::Marketing.:Pancake-omatic.
   “Independent taste tests conducted in Good Housekeeping kitchens have not been able to tell any differences between raw and pasteurized eggs,” according to "Food Safety News", and in two out of three tastings a "Chicago Tribune" reporter preferred pasteurized eggs flavor over farmers market eggs. 
  
  According to "International Business Times", demand for pasteurized shell eggs within the food service industry is strong because, as of 2008, “states such as California, Iowa, Michigan, Wisconsin and Illinois have adopted the most recent FDA Food Code, in which pasteurized shell eggs shall be substituted for raw eggs to at-risk groups.” 
   BULLET::::- An egg beater is a hand-cranked mixing device for whipping, beating, and folding food ingredients. It typically consists of a handle mounted over a piston, which drives one or two beaters. The beaters are immersed in the food to be mixed. In 1856, American tinner Ralph Collier of Baltimore, Maryland, invented and patented the first rotary egg beater with rotating parts. Collier was issued U.S. patent #16,267 on December 23, 1856.
  
  1856 Condensed milk
   BULLET::::- a registration number indicating the hen laying establishment
  
  The egg stamp is required in the EU on all class A eggs unless these are sold directly on the farm.
  
  Section::::Producer code.:Method of production.
  
  The first number of the egg code defines four levels of production quality in decreasing order - the requirements on organic farming eggs extend on the requirements of free range eggs. 
  
  In the European Union these levels have strict minimum requirements:
   BULLET::::- government and industry regulation: A wide range of quotas, marketing boards and legislation governing agriculture impose complicated limits, and often require significant resources to navigate. For example, on the small farming end, in many jurisdictions, there are severe limits or prohibitions on the sale of livestock, dairy and eggs. These have arisen from pressures from all sides: food safety, environmental, industry marketing.
   Section::::Retailers.
  
  Several major retailers have a policy of selling only free-range eggs, or, not selling battery-cage eggs. Some retailers apply this policy not just to eggs in their shells, but also to eggs used in baked goods and processed products such as ready-made meals, quiches and ice cream. In the UK, The Co-Operative and Marks & Spencer sell only free-range shell eggs and use only free-range eggs in their entire range of baked goods, processed products, and ready-made meals. Waitrose sells only non-cage shell eggs, and uses only free-range eggs in their processed products and ready-made meals.
   In February 2013, after repeated presence in front of retail chain Monoprix stores by L214 activists in many big cities of France, the retailer decided to stop using eggs from hens grown in battery cages in their own brand from April 2013, using free-range eggs instead.
  
  Section::::Main awareness campaigns.:Novotel.
  
  In February 2010, after negotiating with L214 during the campaign led against Novotel, the hotel chain announced that eggs on their breakfast menu would henceforth come from free-range hens.
  
  Section::::Main awareness campaigns.:Charal.
  