The eighth commandment of Detective fiction is:
The detective must not light on any clues are not instantly produced for the inspection of the reader.
In his 1939 commentary on his commandments, Fr. Knox said:
Any writer can make a mystery by telling us that at this point the great Picklock Holes suddenly bent down and picked up from the ground an object which he refused to let his friend see. He whispers ‘Ha!’ and his face grows grave – all that is illegitimate mystery – making. The skill of the detective author consists in being able to produce his clues and flourish them defiantly in our faces: ‘There!’ he says, ‘what do you make of that?’ and we make nothing.
I agree with this commandment, though with some reservations.
Before I get into that, I want to mention a fun fact about where the word “clue” comes from: it was originally “clew,” which meant a ball of yarn or thread. It came to have its current meaning from the story of Theseus and the Minotaur, when Ariadne gave Theseus a ball of yarn—a clew—which enabled him to get out of the maze in which the Minotaur lived. A clew was thus, by metaphor, something which enabled someone to get out of a maze of confusion by following it. (The spelling “clue” came about in the 16th century.)
To begin with my agreement: something I’ve talked about before in my discussion of these commandments is the difference between mystery and mere obscurantism. A mystery is a thing which internal consistency. This internal consistency makes it is possible, through learning some of the pieces, to figure out the rest. Mere obscurantism is just a form of “I’m thinking of a number, try to guess which. It was 7.73792555161789434!” There’s no skill involved in defying someone else to read your mind.
This, along with Commandment #6 (accidents), may be one of the most often broken commandments. Writing mysteries is hard, and resorting to cheap tricks is a perennial temptation. That said, there are ways to break this commandment which are not cheating.
The one which comes to mind first is where the clue is mere confirmation of the detective’s theory. It does have to be a theory which is not only supported by the evidence but the only theory which is—otherwise it violates Commandment #6—but as long as the clue is merely confirmation of what the reader should have guessed, its being withheld is only a way of being playful and signaling to the reader that all of the requisite clues have now been shown.
This does (basically) require a Watson character that the detective is encouraging to guess the solution; if the story is told from an omniscient perspective rather than the perspective of the Watson, this is harder to pull off. The author may have to resort to speaking directly to the reader, as Sayers did in my least favorite of her books (The Five Red Herrings), “Here Lord Peter Wimsey told the Sergeant what he was to look for and why, but as the intelligent reader will readily supply these details for himself, they are omitted from this page.”
The other way to violate this commandment—fairly—which I can think of is not very significant, but is probably worth mentioning. There is nothing wrong with the detective withholding clues for a short time. If the detective finds several things and refuses to reveal them in the presence of witnesses but waits until he’s alone with his confidant, there is no harm in this. There is no great benefit either, of course, but it can be used to create some drama because of suspicion falling on whoever it was the detective did not want to see the clue.
It is also possible to separate the finding of a clue and the realization that it was a clue as long as the detective is also in the dark at the time it was found. Supposing that there was a penny on a night stand which was a clue, and it was hidden among some other coins, the detective could pick them all up and put them into a bag without noticing the penny, only to realize that the penny might be significant and then to look into the bag to look at the dates on all the coins present. In a technical sense the clue would have been discovered earlier, but only revealed later. This is fair enough in a mystery novel, so long as the detective reveals the clue when he finally looks closely enough at it to notice its significance.