diff --git a/src/tools/pen.ts b/src/tools/pen.ts
index f22335b..19062ec 100644
--- a/src/tools/pen.ts
+++ b/src/tools/pen.ts
@@ -5,6 +5,9 @@ import { DrawLineCommand } from '../commands'
 import DrawingContext from '../drawing_context'
 import Layering from '../core/layering'
 
+const TENSION_TO_SMOOTHNESS_RATION = 1 / 100
+const THRESHOLD_TO_SMOOTHNESS_RATIO = 10 / 1
+
 export class PenTool extends Tool {
   private line?: Konva.Line
   private layering!: Layering
@@ -18,6 +21,8 @@ export class PenTool extends Tool {
   // line following the user's mouse movement.
   private mode: 'free' | 'line' | 'none' = 'none'
 
+  private smoothness = 0
+
   activate(context: DrawingContext): void {
     super.activate(context)
     this.layering = context.layering
@@ -31,11 +36,7 @@ export class PenTool extends Tool {
   onMouseClick(_: KonvaEventObject<MouseEvent>): boolean {
     if (this.mode === 'none') {
       const { x, y } = this.context.getRelativePointerPosition()
-      this.line = new Konva.Line({
-        ...this.context.overlayProperties,
-        globalCompositeOperation: 'source-over',
-        points: [x, y, x, y],
-      })
+      this.line = this.startLine(x, y, x, y)
       this.layering.overlay.add(this.line).draw()
 
       this.mode = 'line'
@@ -52,11 +53,7 @@ export class PenTool extends Tool {
     if (this.line || this.mode === 'line') return false
 
     const { x, y } = this.context.getRelativePointerPosition()
-    this.line = new Konva.Line({
-      ...this.context.overlayProperties,
-      globalCompositeOperation: 'source-over',
-      points: [x, y],
-    })
+    this.line = this.startLine(x, y)
     this.layering.overlay.add(this.line).draw()
 
     this.mode = 'free'
@@ -88,30 +85,88 @@ export class PenTool extends Tool {
     return true
   }
 
+  onKeyDown(event: KeyboardEvent): boolean {
+    if (event.code === 'Escape' && this.mode === 'line') {
+      this.endLine()
+      return true
+    } else if (event.code === 'KeyS') {
+      this.smoothness = this.smoothness === 0 ? 50 : 0
+      return true
+    }
+    return false
+  }
+
+  private startLine(...points: number[]): Konva.Line {
+    return new Konva.Line({
+      ...this.context.overlayProperties,
+      globalCompositeOperation: 'source-over',
+      points: points,
+    })
+  }
+
   private endLine(): void {
     const line = this.line!
+
     if (this.mode === 'line') {
       line.points().splice(-2, 2)
     }
-
     line.remove()
     this.layering.overlay.draw()
 
-    if (line.points().length > 2) {
-      line.setAttrs({ ...this.context.properties })
-      const command = new DrawLineCommand(this.layering.currentLayer, line)
-      this.context.executor.execute(command)
-    }
+    this.executeCommand()
 
     this.line = undefined
     this.mode = 'none'
   }
 
-  onKeyDown(event: KeyboardEvent): boolean {
-    if (event.key === 'Escape' && this.mode === 'line') {
-      this.endLine()
-      return true
+  private executeCommand(): void {
+    const line = this.line!
+    if (line.points().length <= 2) return
+
+    const extra: { [key: string]: unknown } = {}
+
+    if (this.smoothness !== 0) {
+      // smoothing using tension works nicely with line mode, but in free mode using high values of tension
+      // gives unwanted results.
+      if (this.mode === 'free') {
+        const threshold = this.smoothness * THRESHOLD_TO_SMOOTHNESS_RATIO
+        line.points(collapsePoints(line.points(), threshold))
+        extra['bezier'] = true
+
+        if (line.points().length < 8) {
+          // bezier lines need at least 4 points, otherwise its an empty line
+          return
+        }
+      } else {
+        extra['tension'] = this.smoothness * TENSION_TO_SMOOTHNESS_RATION
+      }
     }
-    return false
+
+    line.setAttrs({ ...this.context.properties, ...extra })
+    const command = new DrawLineCommand(this.layering.currentLayer, line)
+    this.context.executor.execute(command)
   }
 }
+
+const collapsePoints = (points: number[], threshold: number): number[] => {
+  if (points.length <= 4) {
+    return points
+  }
+
+  const ret = points.slice(0, 4)
+  let [lastX, lastY] = [points[0], points[1]]
+
+  for (let i = 2; i < points.length - 2; i += 2) {
+    const [x, y] = [points[i], points[i + 1]]
+    const distance = (x - lastX) ** 2 + (y - lastY) ** 2
+
+    if (distance > threshold) {
+      ret.push(x, y)
+      lastX = x
+      lastY = y
+    }
+  }
+
+  ret.push(...points.slice(-2))
+  return ret
+}