One of the most intriguing tools in Adobe Illustrator is the Spiral tool, not because of what it does – creates spirals – but because of the mysterious terms used its dialog box. Words like Radius, Decay, and Segments leave artists with more questions than answers.

It's one of those tools that, even when you play with it,  little light is shed on understanding exactly how it works.

A few years ago, one bright student asked me the following questions:

“What is this Radius actually measuring and from what center point.”

“What is meant by Decay and why is it a percentage?”

“I know that a Segment is the path that connects two anchors points. But how does changing the number of Segments alter the spiral?

To answer these questions, I had to go beyond just playing with the tool. I had to totally dissect it, one option at a time. And believe me, unlocking the keys to the Spiral dialog box had me going in circles.

To stop this useful tool from spiraling out of control, follow the simple steps below as you solve the mysteries of this intriguing dialog box for yourself.

STEP 1 Opening Up a Mystery.

Select your Spiral tool and click anywhere in your work area to open the Spiral dialog box. Of course, like many of the object creation tools in Adobe Illustrator, you could just “click and drag” to create a Spiral, but then you would not have access to any of the options.

To follow along with me as I uncover clues, fill in the settings shown the dialog box above and click OK. I chose 1" for the Radius because it's the simplest number to deal with and will make it easier to uncover the relationship between all these options. Leave all other numbers at the defaults – Decay at 80% and Segments at 10.

STEP 2 The Radius of What, Measured from Where?

The first fill-in box in the Spiral tool dialog is Radius. If you're thinking ahead, you probably have concluded that Radius is the same as in other drawing tool dialog boxes – the measurement from the center of the object to any of its sides. Well, that was my first thought, and like me, you're wrong.

The Radius is actually the width or height of the first segment. You can prove this by selecting the segment with your Direct Selection tool (white arrow) and just Copy (Edit > Copy) and Paste it (Edit > Paste).

If you check the Info palette or the new Control palette, you'll see that both the width and height (or the Radius) of this segment is 1" measured from the inside of the arc where the width and height intersect in upper left corner of the bounding box.

STEP 3 Decay? Has Our Spiral Died Before We Even Created It?

Stay calm, your Spiral is going to be fine. Decay is just the Radius of each succeeding segment multiplied by the percent that you specify in the dialog box. In other words, if you select the second segment in the Spiral with your Direct Selection tool, then Copy and Paste it, the Width and Height should be 80% of 1" (the Radius of the first segment) or .8". The third segment's radius should measure .8" X 80% or .64", and so on and so on through all 10 segments.

But what if you change the percentage from 80? The closer the Decay is to 100% the more circle-like (or tightly wound) the Spiral will be. When you lower the Decay percentage, the Spiral will be looser. At some point it will not look like a Spiral at all – just a long arching path.

STEP 4 Segments Simply Make Sense.

If you count the segments in your Spiral, you'll see that there truly are 10. But what happens if you increase the number to 20? Well if everything follows what was noted above, the spiral will continue winding further inward toward its center for 10 additional segments, each with a Radius that is 80% of the previous segment.

STEP 5 You Gotta Have Style.

At last there's something that's crystal clear – Style. All it determines is which direction the Spiral winds – clockwise or counter-clockwise.

STEP 6 Straightening Out the Spiral Mystery.

Now that we've solved the mystery of spirals here's a little bit more to think about.

Is a Spiral with a Decay of 100% really just a circle? Nope! It looks like a circle but it's a Spiral that circles straight down over the top of itself.

To see for yourself, create a Spiral with a 100% Decay. Then select any segment or anchor point in the object with your Direct Selection tool and move it. You'll uncover other parts of the Spiral hidden underneath what you just moved.

What happens with a Spiral when its Decay is greater that 100%? No, you don't get an Error Message. Try it yourself and discover a whole new twist in Spirals.

Courtesy of Layers magazine.