Posted by Philip on Jul 28, 2018; 6:12pm
URL: https://forum.librecad.org/Resize-a-rectangle-or-create-rectangle-with-specific-dimensions-tp5716174p5716259.html

Hi FNog and welcome to the forum.

I'm not sure what you mean when you say, "My issue, right now, is how do I select a specific point from a line, polyline, rectangle, etc..."  What throws me is the word "from a line."  If you mean "on a line" then using a Snap will be the most accurate.

Snaps are just what the name implies: snap the cursor to a point on an entity.  The point can be an end point, a middle point, a center point and so on.  An entity can be a line, rectangle, circle, etc.  As an example, you can turn on Snap to End and the cursor will snap to an endpoint of the closest entity.  From there you can use relative addressing to specify a location.
 If using an absolute address then a point on a line makes no difference: 0,0 is absolute regardless of any point in the drawing.  Scroll towards the bottom of the thread I have linked to below and read my description of the Snap icons.

If you are wanting to start drawing an object a certain distance away from an existing object it is easily accomplished.  In this example I will use the 2 Points line command.  Assume that you already have an object on the screen.  Click on Line>2 Points.  You are asked to "Specify first point."  Click on the current object at your given location (end point, mid point...).  You are than asked to "Specify next point."  Right click.  This cancels the second point and you are asked to supply the starting point again.  Then enter the relative address for the starting point of the line.

http://forum.librecad.org/Librecadni10-brief-self-introduction-td5716200.html

I hope this helps.  Ask again if you have any questions.