RE

I see the coordinates. x is left/right y is up down and z is forward backward. Now I see that if I have a square at (50,50,0) and another square at (50,50,20) they would should be exactly on the same spot on screen. But then they can't be on the same spot because this is 3d, so (p, q) (of z=20) will not be 50,50, but something else based on similar triangles and on Z.

Well, that is the way I see it, but I guess im wrong and can't understand it. Thanks for the help, but I'm going to try finding these laws of 3D myself, and hopefully I'll make it to what you were trying to tell me.

SCREEN 12
PRINT "this is where I'm going to start with"
PRINT "rotation"
LOCATE 20
PRINT "Red means on the projection and will be smaller on screen"
PRINT "white means is on the screen and will be bigger"
z = .3
FOR i = 0 TO 50
x1 = COS(i) * 50 * z
y1 = SIN(i) * 50

x2 = COS(i) * 50
y2 = SIN(i) * 50 * z

IF x1 > 0 THEN c1 = 12 ELSE c1 = 15
IF y1 < 0 THEN c2 = 12 ELSE c2 = 15

PSET (x1 + 100, y1 + 100), c1
PSET (x2 + 100, y2 + 100), c2
NEXT

COLOR 10

LOCATE 9, 15: PRINT " this is line z"
LINE (120, 80)-(80, 120), 10