In-betweening in animation: keys, breakdowns, timing and how Blender creates in-betweens

This guide explains the vocabulary first (keys, breakdowns, timing, spacing, ones and twos), then the Blender tools that create or adjust in-betweens. Menu names are those of Blender 5.2 LTS, which moved and extended the In-Betweens tools.

What is in-betweening?

In-betweening, also written inbetweening and often shortened to tweening, is the creation of the intermediate frames between two key poses, so that a movement looks continuous instead of jumping from one pose to the next. An animated action is built in three layers:

• Keys: the poses that tell the story, such as the start, the end and the extremes of an action, like the highest point of a jump.
• Breakdowns: the pose that decides how the motion travels from one key to the next. It is rarely the exact halfway point: it sets the arc, and which part leads or drags behind.
• In-betweens: the remaining frames, which follow the path set by keys and breakdowns.

In large hand-drawn studios, these layers were split between people. The animator drew the keys, often with a timing chart on the drawing: a small scale of ticks showing how many drawings go in between and where each one sits. Assistants usually added the breakdowns, then the inbetweener drew the remaining drawings on the light table, following the charts. In small teams, animators often did the whole process themselves.

In 3D, Blender does the inbetweener's job by interpolating every frame between your keyframes, and the animator's work moves to the keys, the breakdowns and the shape of the curves.

Timing and spacing: what in-betweens control

Timing is how many frames an action takes: 12 frames at 24 frames per second is half a second. Spacing is how far the subject moves from one frame to the next within that time. Two actions with the same timing can feel completely different. Evenly spaced frames give a constant, mechanical speed. Frames bunched close to a key make the subject slow down into it or speed up out of it: slow in and slow out, or easing. In Blender, the interpolation mode and the handles of each key set the spacing, and the Graph Editor shows it: where a curve is steep, the motion is fast; where it flattens, the spacing tightens.

On ones, twos and threes

These terms say how long each drawing or pose stays on screen:

• On ones: a new image on every frame, 24 per second at 24 fps.
• On twos: each image held for two frames, 12 per second at 24 fps.
• On threes: each image held for three frames, 8 per second at 24 fps.

Hand-drawn animation often works on twos, which halves the number of drawings and still reads as smooth for most movements; fast actions tend to go on ones. 3D animation is on ones by default, since Blender evaluates every curve on every frame. Twos come back in 3D by choice, for a stop motion or hand-drawn look. Why a string of still frames reads as motion at all is covered in the guide to persistence of vision.

A worked example: a ball over 12 frames

Take a ball keyed at X = 0 m on frame 1 and X = 12 m on frame 13, at 24 fps. The move lasts 12 frames, half a second, and Blender creates 11 in-betweens. The table gives the ball's position with Linear interpolation, with the default (Bézier, Auto Clamped handles), and with that default on twos (Stepped Interpolation modifier, Step Size 2, Offset 1), as read in Blender with factory settings.

The timing is identical in all three columns; only the spacing changes. With Linear, the ball covers 1 m on every frame and starts and stops at full speed. With Bézier, it covers 0.236 m on the first frame and 1.486 m per frame around the middle, about six times more: two keys with Auto Clamped handles ease in and out on their own. On twos, the ball only moves on odd frames.

How Blender creates in-betweens: interpolation

Every keyframe carries an interpolation mode that controls the segment from that key to the next one. Select keys in the Dope Sheet or the Graph Editor and press T (Key > Interpolation Mode):

• Constant: no in-betweens. The value holds until the next key, the usual choice for blocking.
• Linear: evenly spaced in-betweens, as in the first column above.
• Bézier: the default, smooth in value and speed, with spacing shaped by the key handles.
• Easing (by strength), from Sinusoidal to Circular, and Dynamic Effects: Back, Bounce and Elastic. Ctrl+E picks which end gets the easing: Automatic Easing, Ease In, Ease Out or Ease In and Out.

New keys use Bézier with Auto Clamped handles, set in Edit > Preferences > Animation > F-Curves. In the Graph Editor, dragging a handle reshapes the curve, and so the spacing, without moving the key. The guide to keyframes in Blender covers inserting keys and switching their interpolation.

Pose-to-pose animators often block with Constant keys, add breakdowns, then switch to Bézier and refine the spacing. The walk cycle guide follows that order: poses keyed in Constant, then Bézier. In a walk, the passing pose is the classic breakdown between two contacts.

Setting breakdowns yourself: the In-Betweens tools

These tools build a pose from the keys on either side of the current frame. Since Blender 5.2 they sit in Pose Mode under Pose > Animation > In-Betweens, and they also work on objects in Object Mode, under Object > Animation > In-Betweens. Older tutorials point to Pose > In-Betweens, which is where the menu was up to Blender 5.1, when the tools only worked in Pose Mode.

The shortcuts belong to Pose Mode. In Object Mode, use the menu or the toolbar, where Breakdowner, Push and Relax share one button, the In-Betweens Tools group. To set a breakdown on a character:

1. Key the two poses, for example on frames 1 and 13, in Pose Mode.
2. Select the bones to adjust and move the playhead between the keys, say to frame 5.
3. Press Shift+E and move the mouse left or right: Blender shows the percentage. While the tool runs, G, R and S limit it to location, rotation or scale, B to Bendy Bones, C to custom properties, and X, Y, Z to one axis.
4. Click to confirm (Esc cancels), then key the result with I: the tools only insert a keyframe themselves when Auto Keying is on.
5. In the Dope Sheet, press R (Key > Keyframe Type) and choose Breakdown. The key becomes a small cyan diamond, and since Blender 5.2 Select > Select by Type selects every breakdown at once.

The Breakdowner mixes the values of the two keys in a straight line and ignores the curve. On the ball above, a 50% breakdown on frame 5 puts it at 6 m, where the Bézier in-between sat at 3.111 m: you decide where the breakdown goes, whatever the frame. Since Blender 5.2 the tools also respect X-Mirror in Pose Mode.

The Graph Editor offers the same idea for keys. Select keys between two others and open Key > Blend (Alt+D): Breakdown sets them between their unselected neighbors, and Ease bends them into an S-curve.

In-betweening 2D drawings with Grease Pencil

Grease Pencil was rewritten in Blender 4.3. Interpolate and Interpolate Sequence kept their names, but the Edit Mode feature that paired strokes by selection order was removed, and scripts now call grease_pencil.interpolate instead of gpencil.interpolate.

1. Draw the two key drawings on separate keyframes, then move the playhead to a frame between them. The onion skinning guide shows how to see both while you work.
2. For one in-between, pick the Interpolate tool in the Draw Mode or Edit Mode toolbar, or press Ctrl+E. The horizontal position of the mouse sets the mix: the left edge of the viewport gives the previous drawing, the right edge the next one. Release the left button or press Enter to confirm.
3. To fill every frame at once, use Interpolate Sequence (Shift+Ctrl+E), in the Draw menu in Draw Mode or the Grease Pencil menu in Edit Mode. In the Adjust Last Operation panel, Step sets the frames between generated drawings (1 for ones, 2 for twos) and Type the curve: Linear, Custom, an easing, Back, Bounce or Elastic.

New drawings are created as Breakdown keyframes. Exclude Breakdowns makes the tools interpolate between the surrounding keyframes that are not breakdowns, and in the Dope Sheet, Key > Delete Breakdown Frames removes the generated drawings so you can start over.

Blender pairs strokes by their order, first with first, second with second, and copies a stroke without a partner unchanged from the earlier drawing. In Edit Mode, when both drawings have strokes selected, only those are interpolated and the others are copied the same way. It works best between drawings made of the same strokes in the same order; a new shape, such as a head turning, still needs hand-drawn in-betweens. The Grease Pencil guide covers drawing, layers and keyframes.

Animating on twos in Blender

You can key every other frame with Constant interpolation, which gets tedious on a long shot. The non-destructive way is the Stepped Interpolation F-Curve modifier, which samples the curve every N frames and holds the value in between:

1. In the Graph Editor, select the curves to step, or one key on each of them.
2. Choose Channel > Add F-Curve Modifier > Stepped Interpolation, which adds it to every selected curve. The Modifiers tab of the Sidebar (N) has the same list under Add Modifier and shows the settings.
3. Step Size is 2 by default, for twos; 3 gives threes.
4. Offset shifts the sampled frames. At 0, Blender samples frames 0, 2, 4 and so on; with keys on odd frames, set it to 1.
5. Start Frame and End Frame limit the effect, for example to keep one fast action on ones.

A fast action on twos can also take a smear frame, a stretched in-between that bridges the gap between two poses.

Common mistakes

• No keyframes to slide between: the selected bones or objects have no keys yet.
• The breakdown vanishes when you scrub: Auto Keying was off and the pose was never keyed.
• The last pose is never reached on twos: with keys on frames 1 and 13 and Offset at 0, frame 13 shows the value of frame 12 (11.764 m instead of 12 m in the example). Set Offset to 1.
• Scrambled Grease Pencil in-betweens: the strokes are not in the same order in both drawings, or one was drawn in the opposite direction, which Flip Mode can fix.
• Expecting twos to halve the render: the Stepped modifier only changes curve values. Blender still renders every frame of the range, even when two frames in a row show the same pose.