And here’s an animation of basically what Alan is describing.
One guidline to set the radius size, then inferencing from there.

@Box, thanks for the great animations!
One little note to simplify the steps: the guide isn’t needed.
The first step could be to draw the magenta arc, then type its radius. It keeps being tangent.
Then continue with either the ‘Arc’ tool for rounded tangent corners (your animation) or the ‘Line’ tool (Alen’s previous post) for straight magenta bevels.
Although, I know, some prefere using guides as visual help.
To be honest, I rarely use guides, usually only in animations to show how something works.
Can you explain this in more detail please?
With the Arc tool, click on the first edge to set the first end of the arc.
Move the cursor to the adjacent edge and get the magenta arc.
Move the cursor toward the corner to show the magenta arc. Notice the Measurements box will be labeled “Radius”.
Let go of the mouse and type the radius.
Here you go Cotty, the trick is in the single click to set, and move to reacquire the magenta inference before typing the Radius.

The thing I’m trying to say is that even after finishing drawing a magenta arc you can still adjust its radius. It will remain tangent. But this method can be tricky and not always works. Typing before the third click (as Box shows) is much safer though.
And even after finishing the arc it can still be “tangent-” adjusted through ‘Entity Info’. Hence my note about skipping guide(s). I try to avoid them if possible.
Thank you @DaveR, @Box and @Wo3Dan. I wasn’t able to change the radius after finishing the arc, but with the tangent inference after the second click it works like a charm. A true D’oh! moment for me!
This forum absolutely rocks! I’m not EVEN going to describe the Byzantine process I’ve used for making radiused Corners in the past! The sequence you guys described so well here is at least 10 times faster… Thank you!!
I’m curious how you guys are able to record and include such clear concise and helpful animations for your post replies. Is there a simple answer or a separate forum discussion that describes the process?
Agreed! I don’t even want to know the cumulative # of non-returnable hrs of my life spent using ridiculous, inefficient methods before learning this. Which was just like 10 minutes ago. :p.
I’d like also to add from a newbie perspective if double-clicking the other corners isn’t automagically rounding the corners for you, the below tips may help:
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Practice with a fresh, new rectangle. I had been messing around w/ the one I was trying on, and had used the ‘Weld’ plug-in on it, so the other corners wouldn’t round for me (until I eventually chose ‘Explode Curves’).
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If the rectangle is in a group or component, make sure you’re editing in that group/component. I’ve found when learning that it can be visually deceptive and easy to appear that you’re editing something when in fact you’re outside it’s group/component and not affecting it at all.
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The area you can double-click to round the other corners is extremely large. If you were to visualize splitting your rectangle into four equal quadrants, you can double-click anywhere in each corner’s respective quadrant to automatically round (I initially thought my problem was missing some hotspot location to double-click is why I mention here).