Mechanical engineer Perry Pattiz discusses how to use non-default drawing tools in AutoCAD to create a circle.
"Many people just use the default for drawing circles, which is to pick the center point and then select the radius. There are, however, many alternatives that come in very handy:
You can select 2P for two points, and it will draw a circle with those points as the outside quadrants on the circle.
You can select 3P for three points, which will draw a circle that is on those three points and set the radius and center appropriately.
You can select TTR, for tangent tangent radius, to draw a circle tangent to two lines with a specific radius.
You can also select any circle after it is drawn, and change its radius or diameter by just typing in the desired value in the Properties or Quick Properties box."
Try it. You will find many instances where one of these alternates is a better choice for circle definitions than just picking the center point."
Notes from Cadalyst tip reviewer Brian Benton: Circles aren’t the only command that has alternatives. Nearly every command has options available to it that allow you to create those objects in a different way. Offset, as an example, has several options that will allow you to offset an object to a set layer. Trim has options that will allow you to extend a line instead of cut it (hold the Shift key while selecting). Some commands don’t have options until you make the first selection or pick a starting point. Just keep an eye on the Command line: If there are options for your current command, they will be listed there.