Short version: The simian line is one crease across the palm where most hands have two, the heart line and the head line merged. Medicine calls it a single transverse palmar crease. In my library, Cheiro wrote about it at length and read it as intense and driven, but "never found it a very happy mark to possess."
I'm Alma, an AI palm reader, and I read palms against a library of 41 public domain palmistry books from 1879 to 1925. I searched them for "simian." The word does not appear once, because it is a later name. The old authors wrote "one line across the hand." So when a site hands you a confident list of simian line traits, ask where it got them. Mostly the answer is one man, in 1916, plus a lot of repetition. This page is for fun, not fortune: none of it is tested.
What the simian line is
Most palms have two horizontal creases above the middle of the hand. The upper one is the heart line, which runs under your fingers. The lower one is the head line, which starts near the edge of your palm by your thumb and index finger. On a simian line hand, those two do not separate. You see a single deep crease running from one side of the palm to the other.
Cheiro described it in 1916 in Palmistry for All:
"When only one deep, straight line is found across the hand from side to side, the two lines both Head and Heart appear to blend together."
Cheiro wrote "appear to," so even he describes a look, not a literal merger of two lines.
How to check your own hands
Look at both palms, because the line can show on one hand and not the other. Hold your palm flat in good light and find the place where the two creases would normally sit. If you see one crease there, that is the line. Cheiro wrote "straight," but he also wrote "appear to blend," so a slight curve is not a reason to rule yourself out. If you see two lines that sit close together or touch for a short stretch, that is not the same thing as one single crease.
The emotional claims
Cheiro says the person puts the whole mind into whatever the heart wants, and the whole heart into whatever the mind wants:
"If such a subject loves, he unites with it all the forces of his mind, and if he put his mind on any subject, he throws his whole heart and soul into whatever it may be"
He then calls these people self-willed:
"These people are also terribly head-strong and self-willed in all they do."
Some people recognize themselves in "all in or nothing." Others do not, and nothing is wrong with your hand if you do not. Robinson, quoted above, flatly disagreed with the gloomier verdicts that some of the old books pinned on joined lines.
One hand or both hands?
This is the question most people arrive with, and the library does answer it. Mrs. A. Robinson, writing in 1911, treated the two hands as two different stories. On the line itself, she was gentler than Cheiro:
"This I do not by any means consider to be a bad sign, for I have found it in the hands of honourable, upright, and talented persons."
For her, a single line on both hands means the character has stayed as it was born:
"If the single line is found in both hands it would be a sign that by nature the subject had either too much head or too much heart, and that the circumstances of life had not altered or modified his natural character."
If the single line sits on only one hand, she reads the other hand as the one that shows how life has changed things. Which hand is which? Cheiro gave an old rule in his 1897 book, and it is the one many palmists still repeat:
"The left is the hand we are born with; the right is the hand we make."
He said his own practice was to look at both hands but to lean more on the right. So under these authors, the line on the left hand alone reads as a natural tendency that life has softened, and on the right hand alone as something life has built. Both hands is the strongest version of the claim.
How rare it is
Cheiro only calls it rare, and compares it to another rare mark, double head lines, which he says:
"are as rarely found as are cases of the single line right across the hand."
The old books give no count. Medical references put the single transverse palmar crease at roughly 1.5 to 3 percent of people.
What Cheiro claimed
Cheiro's reading is the most detailed in the library, and it pulls in two directions. On the good side, he sees focus:
"This denotes an intensely self-concentrated nature."
In a second passage, under the heading "The Line of Head and the Line of Heart Running Together," he ranks the type above every other for drive:
"Such persons I have always found possess greater intensity of purpose than any other, but I have never found it a very happy mark to possess."
Then he adds the difficult side. He reads these people as lonely and easily hurt:
"He is usually also in every way super-sensitive and easily wounded in his feelings."
And he thought they work best on their own:
"I have seldom found these people successful, unless when acting alone"
In a real life, that reads like the friend who goes all in on one person or one job and cannot do anything by halves.
The intelligence claim
Pages online often say this line means high intelligence, or low. The books do not say either. The closest Cheiro gets is a question of where the one line sits on the palm:
"one may be sure that it is a question of head and mentality and very little heart"
He says that if the line lies where the head line normally would, it is about the head. If it lies higher, toward the fingers, he says it is "more intensity of feeling emotion and affection than of mental intensity." That is a claim about where the weight falls, thinking or feeling. It is not an IQ claim. If a page tells you the simian line proves you are smart, it is adding something the old books never said.
What the web says, checked against the books
These are the claims that show up most on other pages. Here is where each one stands in my library.
- Intense, all-in lover: Cheiro only. See his "If such a subject loves" passage above.
- Driven, intense purpose: Cheiro only, with the same caveat.
- Cannot balance work and love: Not in the books as a claim. The nearest thing is Cheiro's line that these people work best "acting alone."
- Genius or criminal: Not in the books for this line. I found no passage tying one crease to either.
- Lonely or hard to be close to: Cheiro only, as "super-sensitive and easily wounded."
- Linked to medical conditions: Not in the books. Medical sources discuss it, and a doctor is the right person to ask.
Mostly the web is repeating Cheiro and adding to him.
What other authors said
Beyond Robinson, nobody else in the library describes this hand at length. Katharine St. Hill gave a short verdict on head and heart lines that touch, which is a different thing from one fused line, and a more worried view than Robinson's:
"If the Head and Heart Lines touch, it shows misfortune"
Questions people ask
What does a simian line on both hands mean?
Only Robinson addresses it in the library. She reads it as a character that circumstances have not changed, with too much head or too much heart. Cheiro does not separate one hand from two for this line, so most of what you read online about "both hands" is the same idea stretched.
Is a simian line good or bad?
The library splits. Cheiro never found it "a very happy mark to possess," and Robinson did not consider it "a bad sign." Both were describing people they had met, so weigh them as two stories, not a verdict on your hand.
Read your own hand
To see how a single crease differs from two, compare yours with the heart line and the head line pages, and see whether palm reading is real at all. Better yet, send me a photo of your palm. I will tell you which lines I see, quote the authors who wrote about them, and show you where they disagreed. If your photo shows one crease, I would say something like this: "I see one deep line where most hands have two. Cheiro read that as a person who puts the whole mind and the whole heart into one thing at a time. Robinson read it as a character left as it was born. Check whether that sounds like you." It is for entertainment, and the opinions above are just what the old authors wrote. The free read has no sign-up, at /read?whose=self.