When we want to Scale or Rotate an object, there always needs to be a reference point – i.e., a point about which the rotation takes place or a point from which the scale starts “growing” from. There is a lot of confusion between the Cursor 
and the Object Origin 
. Let’s get it clear:
- Scale and Rotate use the Object Origin
as the reference point about which things are rotated or away from which objects are scaled. - The cursor is used as a location marker.
- If the point about which the object is to be rotated (or scaled from) is not where the Object origin is located, then the Cursor is moved to the correct position and then the Object Origin is moved to the cursor.
- This is achieved with a “Pie” menu called up with the keystrokes
.
Let’s see how this works in practice:
Let’s say we want to rotate this prism by 30 degrees around this corner:
Suppose we just execute the Rotate command as it stands. In that case, the rotation will not only be uncontrolled and mess matters up, but Blender will also try to rotate the prism about the Origin point, which is currently in the middle of the diagonal surface. The Scale command will grow it in all directions using a reference point of the middle of the diagonal surface. We must therefore change our reference point to the corner indicated.
How?
First switch to Edit mode 
. Then click the corner vertex to activate it:
Note: the Origin Point has not moved.
Now let’s move the cursor to that active vertex. (We cannot just click it in Cursor mode).
Type 
and the Cursor/Origin Point pie menu appears:
Note: A “pie” menu is a set of menu options arranged in a circle or oval instead of in a row or column.
The cursor now moves to the activated vertex position:
Now we move the origin point to the cursor. We first go into Object mode and then click Options:
Click again, and the pie menu re-appears:
Now we must switch to ortho view 
before rotating:
Now we can rotate 30 degrees:
And now we scale The prism up 2x:
To Summarize
Move the cursor into position first. Then move the origin (only) to the cursor. Then perform a Scale or Rotate operation in Ortho view.
If the cursor needs to be on a particular corner, first switch to Edit mode so that the vertices are accessible. Select a single vertex. Then and click 
