When people write a letter or a note, it's possible that the pressure of their pen or pencil will create indentations not just on the paper they're writing on, but on any paper that's underneath it, too. In some sorts of story (particularly Mystery Fiction, though not exclusively), making use of this fact is a well-established investigative technique. If you want to know what someone wrote on a notepad but the note is no longer there, just look at the next piece, possibly shading it with a pencil to bring out the contrast. In certain genres and at certain times, this trope has been common enough that a Dangerously Genre Savvy villain might take steps to avoid it, such as by taking several sheets from a notepad to prevent the pesky detective from doing this. A number of variations on this exist--for example, it isn't always paper that the indentation gets made on. A similar concept from the time when writing involved a lot of wet ink was to look at the blotting paper (used to absorb excess ink).
- In North By Northwest, Thornhill is able to figure out where Eve is going by finding the impression of an address she wrote.
- Subverted in The Big Lebowski: The Dude uses this trick to see what Treehorn wrote while taking a phone call, but it turns out to be just a doodle of a man with... implausible anatomy.
- Wild Wild West. Dr. Loveless writes down information about the rendezvous point on a piece of paper and gives it to Bloodbath McGrath. Jim West uses this technique to discover where the rendezvous point will be.
- In M the child-murderer writes a letter to the newspaper, using a single sheet of paper, a pencil and a wide windowsill in his apartment as a desk. When the police raid the apartment, they find a partial impression of the words in the letter in the wood. In this case, they already knew what had been written; the indentations were, instead, proof of who had written it.
- In the novelization of Gabriel Knight: Sins of the Fathers, when Gabriel discovers that Professor Hartridge is dead, he also finds that the office has been picked clean of evidence, including some notes the professor had been working on to help Gabriel. Fortunately there's enough indentation left in the sheets underneath the removed notes that Gabriel is able to use a pencil to employ this trope to find out what the professor had written anyway. (In the game itself, Gabriel is able to just take the notes without having to go through all that.)
- Mentioned by Sherlock Holmes when telling Watson how he got his information as "a fact that has dissolved many a happy marriage". However, in this case he uses the blotting paper to obtain the reverse message, as the writer had used a pen.
- In Les MisÚrables, Valjean discovers Cosette's letters to Marius by looking at the blotting paper.
- Agatha Christie probably has many examples, but there's definitely a Miss Marple short story. Very shortly before his well-off wife dies, a man writes a letter; the rather suspicious phrases "hundreds and thousands" and "when she is dead" are picked up from the indentations on the sheet underneath. The man however explains he was writing a reply to his brother, who had asked for money; whilst he would have money after his wife was dead, he had none currently, and that hundreds and thousands of people were in the same boat. He's lying.
- The psychokinetic children in From the New World are given their own mantra and told to keep it a secret, but the heroine was curious enough to persuade her classmate into writing down his mantra and giving her a glimpse of it. She couldn't see it clearly, of course, but she read it by shading the sheet underneath with a pencil after he left.
- In Bulldog Drummond, a mook assigned to trail Drummond uses this to find out what he wrote in a telegram--only to reveal a rude message from Drummond, who'd realised he was being followed.
- Mentioned briefly in the book version of The Time Traveler's Wife; this is how Clare discovers Henry's intention to get a vasectomy.
- Often mentioned in the City Watch subseries of the Discworld books, but the perpetrator has always had the foresight to tear off several sheets of paper so as to avert this.
- The Dresden Files: In Death Masks, Harry Dresden recovers a thief's notepad after seeing her tear off the top page. He shades the page underneath it with charcoal to find out what she'd written.
- In an episode of Lois and Clark, Clark uses his powers to read several messages indented on a wooden desk.
- In the Eureka episode "Before I Forget" Carter uses this technique on his ticket book in order to learn the license plate number of the last car he wrote a ticket for.
- Found in episode of Shark with two variations: the first being that it's not a text, but a drawing, the second that they don't use a pencil to bring it out, but a computer.
- In an episode of Ashes to Ashes, Alex Drake uses a pencil on Super Mac's Diary to uncover a secret meeting between him and a murder suspect.
- An episode of CSI NY has the motive revealed by this type of clue in the Body of the Week's notebook. The vic had discovered a dextroamphetamine abuse epidemic in his high school and was going to blow the whistle on it to the New York Times.
- In an episode of Monk, while investigating an apparent suicide, Mr. Monk spots indentations on a plastic placemat, and is able to discover the writing, and the truth, with colored chalk.
- In Elementary a piece of newspaper is found at a bombing that killed two people. A word was found indented on one of the pieces of newspaper used to make it and the handwriting on it was matched to the bomber.
- Used by Lee in one episode of The Walking Dead to get instructions for how to drive a train from a ripped notepad.
- Cole uses this trick to gather evidence for a few cases in L.A. Noire.
- In the Creepypasta Easter Egg- Snow on Mt. Silver, the sister learns the redacted gameshark code by holding the paper up to a light to see the indentation.
- Phineas and Ferb has Candace use this when she and Stacey are trailing the boys in England, referencing Sherlock Holmes when they find a notepad with half a page torn off. Candace shades the page underneath to reveal the full message and the location of Phineas and Ferb.
- An old episode of Scooby-Doo has Fred, Velma and Daphne come across someone's diary whilst looking for clues. They find that the ink of the text has faded, but the pressure of the pen has worn through to the next page. So one of them grabs a coal and shades the paper to see what was written.
- Used as a real technique by police, should the circumstances happen to suit it. There are machines called electrostatic detection devices which are able to do a considerably better job the pencil-rubbing method.
- In many typewriters, the ribbon will have the shape of the letters typed clearly visible where ink was pushed off for the letter that was printed. Unrolling the ribbon would allow someone to see what's been typed.
Hello, Unknown Troper. You'll need to get known to lend a hand here.