Sooner or later a drawing needs a long member cut short, and the cut has to be marked with a wave. Most people reach for SPLINE and nudge points until it looks about right, then get a different shape the next day. A break line in AutoCAD can be drawn four quite different ways, and only two of them repeat exactly.
This article draws all four in one file and compares them. For the whole discipline of presenting a drawing set to standard, the Institute runs an AutoCAD course for construction.
What the wavy line actually means on a drawing
Before choosing a method it helps to know the job, because each job wants a different shape.
On a technical drawing the wave is a thin continuous line that bounds a region removed from view. A member too long for the sheet is cut short in the middle and the two halves are closed with waves. The same line also separates a view from a local section.
Being a thin continuous line, its width is about half that of a visible outline. It is not decoration, so the shape should be even and identical everywhere in one drawing set.
Note that a wavy line in autocad is never a special object type. Whichever route you take below, what lands in the drawing is an ordinary spline, polyline or line.
That is exactly why nudging points by hand does not do. The full symbol table for construction drawings is in our article on construction drawing symbols.
No dedicated command, but four routes
One thing should be said plainly. There is no single command that draws a wave the way CIRCLE draws a circle.
There are four routes instead, each strong somewhere different.
| Route | What you get | Repeats exactly? |
|---|---|---|
SPLINE through staggered points | A soft wave that bulges past your points | Only if you type coordinates |
| Polyline with ±1 bulges | Every half-cycle is an exact semicircle | Yes, completely |
BATTING linetype | A chain of loops running the whole length | Yes, by scale |
BREAKLINE command | A straight line with a break symbol in it | Yes, completely |
How to draw a wavy line with SPLINE
This is the method everyone knows, and the only one the other sites mention.
Type SPL, then pick points alternating either side of an imaginary centre line. Evenly spaced points give an even wave. Press Enter twice to finish.
One detail goes unnoticed. A Fit spline bulges past the points you place. Points offset 50 units each way, so 100 apart, produce a curve that measures 102 units tall. Those extra 2 units hardly matter on a break line, but they matter when a curve has to touch a known mark. How Fit and CV differ is covered in our article on the SPLINE command.
The real weakness is that picking with the mouse cannot be repeated. Typing coordinates repeats perfectly, but nobody types eleven coordinate pairs twice.
Polyline bulge: a mathematically exact wave
This route is less known and gives the cleanest result, because it relies on geometry rather than on a steady hand.
A polyline can curve segment by segment. That curvature is called polyline bulge, and a value of 1 turns the segment into exactly half a circle. Alternate +1 and −1 along the segments and you get a chain of flipped semicircles, which is a perfect wave.
The numbers prove it. Ten segments with a chord of 100 measure exactly 1,570.80 units over a straight run of 1,000. The ratio 1.5708 is π/2, precisely the ratio of a semicircle to its diameter. The wave measures 100 tall, neither more nor less.
By hand, type PL, choose Arc, then flip the curve direction each half-cycle. A short LISP routine that takes two ends and a cycle count is far quicker. The Institute's own tools are described in our article on an AutoCAD LISP library.
The BREAKLINE command: already installed, rarely used
This is where every other page stops short, even though the tool is already on your machine.
AutoCAD ships with Express Tools, and one of them exists purely for this. Autodesk describes the breakline command in a single line: "Creates a breakline, a polyline that includes the breakline symbol." So it produces one polyline with the symbol built in, not a separate block dropped on top.
Using it takes three picks. Type BREAKLINE, pick the first point, pick the second, then press Enter to place the symbol at the midpoint. Pick a third point instead of pressing Enter to put the symbol somewhere else.
The size is not set directly; it follows DIMSCALE. Autodesk's BREAKLINE page states it outright: "The current setting of the DIMSCALE System Variable controls the ultimate size of the breakline elements." Set DIMSCALE to 100 on a drawing plotted at 1:100 and the symbol comes out the right size straight away.
The first prompt offers Block, Size and Extension. Size multiplies the symbol, and Extension is how far the line runs past the two points you picked, exactly like DIMEXE on a dimension.
Replacing the breakline symbol with your own wave
This is the most valuable part of the command, and the part almost nobody writes about.
The default breakline symbol is a zigzag, not a wave. Many people see that and abandon the command, when the symbol is replaceable. The default lives in brkline.dwg in the AutoCAD Express folder, and it holds just three objects: one polyline drawing the zigzag, plus two points.
Those two points are the whole trick. They sit on the DEFPOINTS layer, exactly 1.000000 units apart, and AutoCAD uses them both to align the symbol along the line and to scale it. Your own block must carry the same two points, also on DEFPOINTS.
The final size follows the formula A × S × D given in the command's own file, where A is the distance between the two points in the block, S is the Size setting and D is DIMSCALE. The default block has A equal to 1, so a Size of 0.5 with DIMSCALE at 48 puts the points 24 units apart in the drawing.
So the recipe for a house-standard break line has three steps. Draw the wave you want, add two points on DEFPOINTS one unit apart, then run BREAKLINE and use Block to point it at your new block.
The BATTING linetype: a wave along the whole line
The fourth route draws no geometry at all. It changes the linetype, so an ordinary straight line comes out wavy.
The acad.lin file shipped with the 2027 release holds 45 linetypes. Only two of them give a wavy shape: BATTING is a chain of loops and ZIGZAG is a sawtooth. BATTING exists for insulation, but it serves as a decorative wave as well.
The thing to know is that the pattern is far too small to see at default settings. One BATTING cycle is only 0.4001 drawing units at LTSCALE 1, and ZIGZAG is 0.8001. On a millimetre drawing at LTSCALE 1 the line looks completely solid.
That gives you the number to set directly. For an 80 mm cycle, LTSCALE must be 80 divided by 0.4001, so about 200. For 40 mm, set 100. Choosing LTSCALE for a whole drawing is covered in our article on the LTS command.
There is one more trap when you apply this to a multi-segment polyline. The PLINEGEN variable defaults to 0, and at that value the pattern restarts at every vertex, so the wave breaks at each corner. Set PLINEGEN to 1 before drawing and new polylines carry the pattern straight through.
Which route for which job
The four are not interchangeable. Each suits a different task.
| The job | Use |
|---|---|
| Cutting members short, done often | BREAKLINE with your own symbol block |
| Bounding a local section, free shape | SPLINE |
| An exact wave you need to measure | Polyline with ±1 bulges |
| Insulation, or a long decorative line | The BATTING linetype |
If one method has to serve a whole office, choose BREAKLINE with a house block. It gives an identical shape on every drawing, the size follows DIMSCALE on its own, and a newcomer only has to remember one command name.
Three things you often hear that are not true
| Often heard | What the 2027 release actually does |
|---|---|
| AutoCAD offers nothing for break lines | BREAKLINE ships in Express Tools |
BREAKLINE inserts a block | It creates one polyline with the symbol in it |
Applying BATTING shows a wave at once | One cycle is 0.4001 units, so it reads as solid until LTSCALE goes up |
Frequently asked questions
What is the quickest way to draw a break line in AutoCAD?
BREAKLINE from Express Tools, since it takes two picks and an Enter. The default symbol is a zigzag; swap the block to get a wave.
How do I make two break lines identical?
Do not draw them by hand. Use BREAKLINE with the same block, or a polyline with ±1 bulges over the same chord length.
What bulge value gives a semicircle?
Exactly 1. Check it by length: a chain of semicircles measures π/2 times the straight run, so 1,570.80 over 1,000.
Why is the line still straight after I applied the BATTING linetype?
LTSCALE is still too small. One cycle is 0.4001 units at LTSCALE 1, so a millimetre drawing needs roughly 100 to 200.
Why does the wave break at the corners of my polyline?
Set PLINEGEN to 1 and draw it again. At 0 the pattern restarts at every vertex.
What must a custom symbol block contain?
Two point objects on the DEFPOINTS layer. AutoCAD uses them to align and scale the symbol, and the command refuses a block without them.