for loop

r_x = 10 
r_y = 10 
For x = 1 To 20 

Rect r_x+(20*n),r_y+(20*n),50,50,0 

Next

Ok. You appear to be a newbie to the extreme. So lets look at this code step by step.

r_x = 10 
r_y = 10 

Those two lines I think you know. They set the variables r_x and r_y to an initial value of 10 each.

For x = 1 To 20 

This is the beginning of your loop. It initializes and defines how you want the loop to act. It initializes variable x to start at a value of 1 and through each iteration of the loop, increment x by one through value 20.

Next

That determines the end of your loop and sends the code back to the For statement to increment the x variable.

For x = 1 to 20 
  Print x 
Next

would print

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

Since x is being incremented each time through the loop, you can use it's value for your rectangles to increment their x and y locations.

Rect r_x+(20*x),r_y+(20*x),50,50,0 

This line is your meat and potatoes of the loop. It draws your rectangles.

r_x+(20*x) is your x location and
r_y+(20*x) is your y location

r_x+(20*x) and r_y+(20*x) on the first iteration would equal 30 (r_x = 10 : 20 * 1 = 20... 20 + 10 = 30)

So on the first iteration it would draw a rectangle 50 pixels wide and 50 pixels high at location 30,30.

r_x+(20*x) and r_y+(20*x) on the second iteration would equal 50 (r_x = 10 : 20 * 2 = 40... 10 + 40 = 50)

So on the second iteration in would draw at 50,50 and so on through iteration 20 where it would draw the rectangle at 400,400 (10 + (20 * 20)).

That should, I hope clear up the whole loop + rectangle mystery. :)

Kanati

[EDIT] And I agree totally with CS_GUY... don't use _ in variable names. It makes them rather unreadable.