There’s nothing weirder than learning that one of your favorite stories didn’t really happen that way. For the thousands of devoted fans of the Little House on the Prairie series by Laura Ingalls Wilder, that problem is particularly acute. After all, the books are based on real events—but events that are also largely fictitious. Where does history end and fiction begin? On the 150th birthday of Laura Ingalls Wilder, here’s a reminder that truth and fiction don’t always coincide.

1. THE INGALLS FAMILY DIDN'T ALWAYS HEAD WEST

From the moment the Ingalls family sets out in their wagon and leaves the Little House in the Big Woods, the Little House books show an unceasing push West. Real life and Manifest Destiny don’t always line up, though, and in fact the Ingalls family tracked back and forth several times before setting down in De Smet, South Dakota.

The Ingalls family’s first stop after Wisconsin was Independence, Kansas (with a possible stop in Missouri), where they built a “little house” on the open prairie. But the land was not theirs to settle: It was owned by the Osage people [PDF] and the Ingalls family, like thousands of other settlers, were squatters waiting for the Osage to be driven out so that the United States could take it over. It’s not entirely clear why the Ingalls family left, but instead of continuing west they went back to Wisconsin.

Next, they went west again, this time settling near Walnut Grove, Minnesota. Then, financial difficulties, illness, and a plague of locusts forced them to move on. They went to visit family elsewhere in Minnesota, but while there, Laura’s 10-month-old brother, Freddie, died after a sudden illness. Then they continued on to Burr Oak, Iowa, where they ran a hotel. The Ingallses then backtracked to Walnut Grove, where Mary lost her vision, then went west again and eventually settled in what is now South Dakota.

Nonetheless, Laura and her daughter Rose Wilder Lane, who heavily edited and helped develop the first books, decided that the fictional Ingallses should always move West. The result is a sense of wanderlust and movement that gives the series its structure.

2. JACK, LAURA'S DOG, DIDN'T LEAVE KANSAS

Aw, Jack! Laura’s happy little puppy pal! Though faithful Jack tracks the fictional Laura through the books until she becomes an adolescent, Laura revealed in Pioneer Girl, the original autobiography that formed the basis for the books, that he was actually left behind in Kansas when Pa traded him for some horses and ponies. When writing the book, Laura decided to have Jack die peacefully in his sleep—perhaps in a way she could control, as opposed to the uncertain fate of her real-life dog.

3. MARY INGALLS PROBABLY DIDN'T HAVE SCARLET FEVER

Everyone knows the story of how Mary Ingalls contracted scarlet fever and lost her sight permanently. Except she probably didn’t. Dr. Beth Tarini, a professor and pediatric and adolescent medicine specialist, obsessed over Mary’s diagnosis from the time she was a child, then discovered in med school that scarlet fever can’t blind someone.

But viral meningoencephalitis can—and Tarini thinks that Laura and her daughter, Rose, attributed the blindness to scarlet fever either to make the story more accessible to kids or because the disease may have already been familiar from other novels like Little Women. She even published an academic paper about it [PDF]. (In real life, Laura wrote Mary had "spinal meningitis," which she crossed out and replaced with "some sort of spinal sickness. I am not sure if the Dr. named it.” She also wrote that the blindness was caused by a stroke, but Tarini deemed a stroke unlikely since there were no other signs of one.)

Mary may not have gone blind from scarlet fever, but she did lose her sight. She ended up attending the Iowa College for the Blind, where she could take classes like civil government, botany, and piano tuning. Mary was an adept student and put the industrial training she got there to good use: After Pa died, she made fly nets to help the family earn more money.

4. THE INGALLS FAMILY HAD GUESTS DURING THE LONG WINTER

Houses were small in the pioneer era, but that didn’t mean that they were all devoted to single-family living. During the “hard winter” of 1880-81, the Ingalls family took in a couple named Maggie and George Masters. George was the son of a family friend and Maggie was his new wife, who had married him in an apparent shotgun wedding situation. “Maggie didn’t want the baby to be born at her folks’ and disgrace them,” Laura wrote in a letter to her daughter, Rose. “George’s folks were mad because he married her.”

The Masters family were not the best of houseguests. In her notes to Pioneer Girl, Ingalls scholar Pamela Smith Hill explains that Maggie had her baby in the house without the assistance of a doctor, and the newlyweds ran out of money and kept wearing out their welcome. “Times like this test people,” wrote Laura, “and we were getting to know George and Maggie.”

So why aren’t they in The Long Winter, the award-winning book that relates the story of a winter so extreme, the blizzards lasted six months? Chalk it up to authorial savvy: Laura felt it would dilute the power of a family stuck inside their house, forced to face the elements as a unit.

5. NELLIE OLESON WASN'T A REAL PERSON

If there’s a villain in the Little House books, it’s Nellie Oleson, the snooty brat who torments Laura when they're girls and tries to steal Almanzo from Laura when they're young women. In reality, though, there wasn’t a single Nellie Oleson. She is thought to have been a composite of three real-life people named Genevieve Masters, Nellie Owens, and Stella Gilbert. Laura had unpleasant run-ins with all three, interactions she apparently never forgot.

BONUS: “ALMANZO” WASN'T PRONOUNCED AL-MAHN-ZO

Laura gives her beau, Almanzo, a sweet nickname in the books: Manly. (She also referred to him as “the man of the place” in real life.) That’s for good reason—his name was pronounced Al-MAN-zo, not Al-mahn-zo.

The wrong pronunciation apparently took hold through the confounding influence that was the Little House on the Prairie TV show—a polarizing pop culture phenomenon that also introduced inaccuracies and anachronisms like adopted children and basketball into the fictional Ingalls family.