AutoCad :: Starting Point Relative To Another Point

How to choose my starting point to be relative to another point without drawing help lines (which mean they must be drawn, selected, deleted = time).

For argument's sake, consider I am attempting to draw a cross. I use the line command, input a starting point; choose next point: @100<0 (to draw a line 100 units in length at an angle 0 [relative to X axis] relative to previous point).

Now I need a vertical line of equal length to cross the middle-point of the horizontal one.
I could snap to middle-point, draw a line of half-length at angle 90 and draw another of 2.(length) at angle -90. This produces 2 lines one of which is unwanted must therefore be selected and deleted.

This simple example illustrates my point.

I need a command which allows me to choose my first point (for the LINE command) as the point which is "X" units at angle "alpha" relative to a point "x,y" (or a coordinate selected by the SNAP tool if you will).