If there's one thing that mystery stories seem to have in common, it's clues. Sometimes, the clues are obvious; sometimes they're not. They may be physical, or they may be psychological, but they are all critical to a good mystery. Mystery lovers seem to always look for clues, and they do not like it (Well, the ones I know don't like it) if the murderer comes "out of nowhere" with no clue to his or her identity.
In the days of Arthur Conan Doyle, clues were physical. They were things such as footprints, cigarette ash and mud splashes. Doyle's Sherlock Holmes was an authority on cigarette ash and the mud from different sections of London, among other things.
By the time Agatha Christie was writing, our ideas of what "counted" as a clue had changed. In fact, in The Murder on the Links, Hercule Poirot bests Inspector Giraud because, while Giraud is looking for those obvious clues, Poirot pieces together what they mean. He also finds clues that Giraud misses, because Giraud isn't looking for them. In several of Christie's novels, there are obvious clues, but if they're not interpreted correctly, they lead the reader in the wrong direction. I don't know for sure, but I would guess that Christie was sending a message to look beyond the obvious. Her sleuths always did.
Sometimes, the clues aren't physical at all. In Cards on the Table, there are really no physical clues as to which of the four people in a parlor killed their host after a dinner party. It is the psychological clues that lead Poirot to the true solution of the story. Christie herself noted that this wasn't a "typical" murder mystery. It isn't. It's unique.
In the last few decades, physical clues have chaged from footprints and fingerprints to other kinds of clues. For instance, in Robert Goldsborough's Murder in E Minor, the clues are found in things people say, in the murder victim's past, and in the identity of one of the suspects. This novel, a continuation of Rex Stout's Nero Wolfe series, uses more modern information-gathering such as identity documents. Most modern mystery novels use such information-gathering unless, like Alexander McCall Smith's No. 1 Ladies' Detective Agency series, the culture and setting require other kinds of clues.
In my Joel Williams series, I use some modern technology to reveal clues, For instance, in B-Very Flat, Joel Williams uses the Internet to locate some information that's critical to finding out who has murdered a young, gifted violinist. Surveillance software is involved in the solution of that mystery, too. Computer technology also figures in the solution to the murder of a graduate student in Publish or Perish.
Very often, the sleuth finds the clues one by one. This happens in John Alexander Graham's Something in the Air. In that story, Professor Jake Landau searches for clues to the sudden death of a good friend of his. As the search goes on, he finds those clues, but they're not given to him all at once. Robin Cook does something similar in many of his novels. In many of his Laurie Montgomery/Jack Stapleton novels, the clues to the mystery come in medical coincidences and other small facts that the sleuths piece together. Finding clues one by one can add to a story's suspense.
Clues are not easy to create and write about as it may seem. The author has to be sure they don't point too obviously to the murderer right away unless the whole point of the novel is the cat-and-mouse game between sleuth and murderer. On the other hand, clues have to be obvious enough that the reader can notice them and, if he or she is paying attention, put them together. They can't be so hard to find that the reader is left guessing at which suspect is the murderer. I've never liked novels where I end up thinking, "Well, of course if I'd known *that,* I would have known who did it!"
What sort of clues do you notice? Do you prefer novels where you are given all the clues and have to figure out from them what happened? Or do you prefer the kind of novel where the sleuth uncovers the clues one by one?