Keyframes and In-Between Frames Explained

What a keyframe actually is

A keyframe is a drawing that carries an idea. It is the pose you would choose if you were only allowed to show one image from the whole action: the peak of the jump, the deepest point of the crouch, the moment the hand closes around the handle. Everything else in the shot exists to get you from one of these drawings to the next.

That definition matters because software has blurred it. In most motion-graphics tools a "keyframe" is just a numeric value pinned to a point in time, and the computer invents the frames in between. In hand-drawn animation the word keeps its original meaning: a keyframe is a complete drawing made by a person, chosen because it communicates something. Frame-by-frame work has no interpolation to fall back on, so the quality of your keys is the quality of your shot.

Classical production splits drawings into three jobs, and it is worth keeping those names in your head even when you are the only person on the shot:

Straight ahead vs pose to pose

There are two honest ways to make a sequence of drawings, and they fail in opposite directions.

Straight ahead means drawing frame 1, then frame 2, then frame 3, discovering the action as you go. It produces motion with a loose, surprising quality that is very hard to plan for — fire, smoke, water, a character losing control. Its weakness is drift: by frame 30 the character has usually grown, shrunk, or wandered off the position you needed them to land on, and because every frame depends on the one before it, fixing frame 10 means redrawing everything after it.

Pose to pose means drawing the extremes first, then the breakdown, then the in-betweens. You know where the action ends before you commit to how it gets there, so the timing stays adjustable and the volumes stay consistent. Its weakness is stiffness: a sequence that is planned too tightly can feel like a series of poses with filler between them rather than a single continuous movement.

In practice almost every shot is a mix. Plan the body pose to pose so the character arrives where the story needs them, then animate the loose parts — hair, a cape, a tail, a trail of dust — straight ahead on their own layer so they keep that unpredictable quality. Working this way is a large part of why separating your drawing onto layers is worth the setup cost.

Breakdowns: the frame that decides the motion

Take two identical keys — an arm down and the same arm up — and you can produce completely different movements from them purely by choosing where the middle drawing sits. Put the breakdown close to the "down" key and the arm snaps up at the end, which reads as effort and weight. Put it close to the "up" key and the arm leaps off its start and eases into position, which reads as light and quick. Put it exactly halfway and the arm travels at a constant rate, which reads as mechanical.

The breakdown is also where you add the things that make a drawing feel alive rather than interpolated. A hand that lags slightly behind the wrist. A head that tips a few degrees off the straight path between the two keys. A shoulder that leads the arm. None of that is visible in the extremes, and a true halfway drawing will never contain it, because a true halfway drawing is just an average of two things you already drew.

How many in-betweens do you need?

Work backwards from time, not from drawing count. Pick how long the action should last in seconds, multiply by your frame rate, and that total is your frame budget — keys included.

At 12 frames per second, a half-second gesture is six frames. At 24 frames per second the same gesture is twelve. The move is identical; the second version simply has twice as many drawings doing the same job, which is exactly why choosing a frame rate before you start saves so much rework. Some useful anchors to plan against:

• A snap or impact — 1 to 2 frames. Often there are no in-betweens at all; the two keys touch, and the hard cut is the point.
• A quick gesture, like a head turn or a point — 3 to 6 frames at 12 fps.
• A deliberate action, like lifting something heavy — 10 to 18 frames, with the extra drawings concentrated at the start and end rather than spread evenly.
• A slow settle — as many frames as you can stand, all crowded near the final pose, so the character comes to rest instead of stopping dead.

More in-betweens never make a move faster. They make it smoother, and they make it longer. If a move feels sluggish, the fix is usually to delete drawings, not add them.

Reading and writing a spacing chart

A spacing chart is the plan for where your in-betweens sit. Traditionally it was drawn in the corner of a key drawing as a short ladder of numbered ticks: the two keys at the ends, the breakdown somewhere in the middle, and the in-betweens placed closer to whichever end should feel slower. You can sketch the same thing on a scrap layer, or just keep it in your head once the habit forms.

The three charts above use the same number of frames and the same two keys. They produce three different performances. This is the whole of what animators mean by spacing, and it is covered in its own right in timing vs spacing.

A pose-to-pose workflow in FrameAnim

The editor is built around this order of work. Add your frames, draw the extremes on the ones that matter, and leave the gaps empty until you have decided the timing. Onion skinning shows you the neighbouring drawings as faint ghosts while you work, which is what makes in-betweening by hand practical at all — you can see both keys while you draw the frame between them.

1. 1
   Draw the two extremes

   Put the start pose on your first frame and the end pose on a frame far enough away to hold the whole move. These two drawings define what the action is, before any of the smoothing exists.
2. 2
   Scrub between them

   Flip back and forth between the two extremes. If the pop between them does not already read as the action you intended, fix the extremes now — no amount of in-betweening will rescue a weak pair of poses.
3. 3
   Add the breakdown

   Draw one frame roughly in the middle. Do not place it halfway between the two drawings: place it where the action naturally lingers or dips, because that choice is what gives the move its character.
4. 4
   Chart the spacing

   Decide, before drawing anything else, which frames will be crowded and which will be spread out. Crowded frames read as slow, spread frames read as fast.
5. 5
   Fill the in-betweens

   Draw the remaining frames with onion skinning on, following your chart. Each in-between is a servant of the keys; it should add no new ideas of its own.
6. 6
   Play it back and adjust

   Play at your target frame rate. Fix timing by adding or removing in-betweens rather than by redrawing the keys, which should stay as the anchors of the shot.

Common mistakes

• Drawing the in-betweens first. Starting at frame 1 and working forward without extremes is straight-ahead animation whether you meant it or not, with all of its drift.
• Treating every drawing as equally important. If all your frames get the same amount of attention, your keys are under-worked and your in-betweens are over-worked.
• Placing the breakdown exactly in the middle. The most common cause of technically clean animation that still feels dead.
• Fixing timing by redrawing keys. Change the number and position of in-betweens instead. The keys are your idea; the in-betweens are your dial.
• Adding frames to fix a slow move. Smoothness and speed are different properties. A sluggish action usually needs fewer drawings, held longer in the right places.

Where to go next

Keys and in-betweens are the vocabulary; spacing is the grammar. Read timing vs spacing next, then apply both to a real exercise in the bouncing ball tutorial, which is the shortest path from theory to a move you can actually watch.

If you are still getting comfortable with the mechanics of drawing frame by frame, start with frame-by-frame animation basics and onion skinning. When you are ready for a long sequence built entirely from planned keys, the walk cycle is the classic test.