This has got to be the easiest way to create a block out of existing linework. The first step is to select some linework:
Then right-click and hold anywhere you want the base point to be until you see a cursor that looks like this:
You could right-click and hold on a grip or anywhere else – it doesn’t matter.
Move your cursor while holding down the right button on your mouse and you will see that AutoCAD copies the linework.
Let go of the right mouse button and AutoCAD will present you with a short menu.
Simply select “paste as block” and your new block will be created. The base point is the place you selected with your mouse.
UPDATE: Then, as one of my subscribers pointed out, use the RENAME command to give the block a logical name. Thank you for pointing this out Tommy!