In France, right up into the early 1800s, wheat, rye and Maslin production were measured as three separate crops. The European Union still lists Maslin as a crop category today.
American-style Pumpernickel Bread is a modern example of a bread made from both rye and wheat, though it’s made from refined rye and wheat flours, made from the grains grown separately, not together in the same field.
The mixed straws from the Maslin are considered excellent animal feed.
Oats and barley grown together are called “dredge.”
Some people say that a mixed crop like Maslin could have been more reliable than a single-grain crop: if the season was colder, the rye would flourish; if the season were hot, the wheat would flourish. However, the converse of that would mean that unless the growing weather turned out to be an exact balance of the two temperature ranges, then one of the grains would suffer, and so the overall yield from that land would be lower.
Their “more reliable” theory makes it sound like everyone at the time would want to grow Maslin. In the Middle Ages, however, what type of grain crop was grown really depended on what the soil and climate combined in the area were best suited for, balanced with what would fetch the best price on the market at the time. [Ed: the same as today, naturally.] In the Little Shelford area of Cambridgeshire, from 1323 to 1348, Maslin was the crop of preference, followed by barley. In some areas of Lincolnshire such as Skirbeck, Maslin was also the most important crop. 
In 1300, on the priory manors of Norwich Cathedral in Norfolk, England, only an estimated average of .75% of arable acreage from the years 1291 to 1370 was used for Maslin. This compared to 53% for barley, 12.53% of wheat, 11.48% for oats, 8.58% for rye, 12.38% for field peas, .16% for beans. 
The proportion of rye to wheat in a Maslin mixed crop would vary. In the “Theatre d’agriculture et mesnage des champs” by Olivier de Serres (1600), de Serres describes two types of Maslin flour: a grey one which was mostly wheat, and a darker one, which was mostly rye. The greyer one was of course considered better.
Records of the time seem to show that farmers would tend to grow either rye or Maslin, but not both. They also show that grain crops provided two important things: the actual grain, for human food and for next year’s crop, and straw, for animal feed and for building purposes (bricks, thatched roofs, wall insulation, etc.)
It’s interesting to note that Maslin wasn’t just a matter of one variety of rye and one variety of wheat growing intermixed; there would have been several varieties of each. The grain fields of the time were very genetically diverse, unlike today’s monoculture of genetically uniform grain fields. Seed saved from fields in an area, generation after generation, century after century, eventually formed a collection of grain varieties now referred to as “land race” mixtures. Consequently, the wheat stalks would have varied in height, and in ripening time. The genetic hodge-podge ensured that, even in a very bad growing season, at least some would survive to provide seed for the next year.
In the 1800s, scientific plant breeding knowledge and methods allowed the development of genetically specialized varieties, focussing on those with much higher yields of grain than the land race mixtures could provide, and with straw just seen as a byproduct, as specialized livestock feeds came on the market to replace it as animal food. One side effect of this was that getting straw long enough to repair traditional thatched roof in England became hard to procure, and is now a specialized taller crop, harvested using older threshing machines that don’t shred the straw.
Some modern writers assume that only the poor would have eaten bread made from Maslin flour, as white bread has always been considered the most desirable. However, there’s far too much written evidence that many people, rich or not, actually liked the taste and texture of bread made from Maslin, and would have it amongst the breads that rotated through their kitchens.
That being said, Maslin bread would probably have been the very top range of bread that the poor could aspire to. Otherwise, their bread (if any at all) would have been mostly all rye, in the style of today’s traditional authentic German Pumpernickel, or made from other grains.
That being said, some researchers think that even saying “the poor ate Maslin or rye bread” may be wildly overestimating the quantity of grains that the poor of the time in areas such as England were even able to aspire to eating. In the documents that historians have access to, historians are mostly limited to monastic documents because that’s who produced documents back then for the most part, and monks were richer than their surrounding peasant neighbours. It’s likely, some researchers surmise, that the poor’s diet was more reliant on legumes such as dried peas and beans, with just a very small amount of grain in the diet. The serfs could grow legumes on their own small private plots which weren’t subject to tithes and rents, whereas grains in fields would be. One wants also to bear in mind that baking raised breads required using the ovens of the estates that the peasants lived on, and a tithe of usually 1/10th of the bread baked was required for using the ovens. Grains cooked into a pottage or porridge, or cooked on a griddle as flat cakes, side-stepped the cost of baking proper loaves of bread.
Literature & Lore
“Maslin cakes. These are similar to the wheaten cakes in preparation except that about a sixth of the grain used is rye. Maslin is an old English word for a mixture of wheat and rye. Modern farmers sometimes grow them together so that the stiff rye straw may hold up the weaker wheat. Modern mills can easily separate the grains; but in the Middle Ages they were ground together and Maslin bread was a staple. Rye gives fewer calories than wheat, and for that reason forms of bread containing it have become popular with those who wish to be slim; and, though it is poorer in vitamins, its pleasant taste suggests the presence of other minerals than wheat has.” — Picton, Lionel Jas. Holmes Chapel, Oct. 31. Writing to” The British Medical Journal. 6 November 1937, page 939.
“Maslin” comes from an old French word meaning “mixed.” The word continued to evolve in French, and became “mesclun”; as in the salad mix now called “mesclun.”
Maslin is also referred to in writing as mixtura, mixtilio, and maislin.
 Hallam, H.E. and Joan Thirsk, Eds. The agrarian history of England and Wales, Volume 2; Volumes 1042-1350. Cambridge University Press, 1989. Page 293, 307.
 The soil in the area was particularly suited for barley; the national average for wheat at the time was 36%, so the Maslin acreage nationally may have been a bit higher, too. Slavin, Philip. Feeding the Brethren: Grain Provisioning of Norwich Cathedral Priory, c. 1280-1370. Thesis for Centre of Medieval Studies, University of Toronto. 2008. page 61.
Clitherow, Mark. Smoke-blackened Thatch. 2 September 2008. Page 5. Retrieved April 2011 from http://www.historicthatch.org/index.php?option=com_content&view=article&id=89&Itemid=111&limitstart=4
Forrest, Alan I. Reshaping France: town, country, and region during the French Revolution. Manchester University Press ND, 1991. Page 58.
Levine, David. At the dawn of modernity: biology, culture, and material life in Europe after the year 1000. University of California Press, 2001. Page 184.
Slavin, Philip. Feeding the Brethren: Grain Provisioning of Norwich Cathedral Priory, c. 1280-1370. Thesis for Centre of Medieval Studies, University of Toronto. 2008. page 59.