> One part that confused me is how I needed to use let() to define startColumn and startRow inside the loop. I don’t understand this… That's because you wrote

> One part that confused me is how I needed to use let() to define startColumn and startRow inside the loop. I don’t understand this… That's because you wrote a for-loop without braces, so it can only contain a single statement, and that single statement is your let(). Here is an equivalent way to write that section of code (and I did test it): for (r = [ 1 : numRows ]) { startColumn = ((c * thicknessWall) + ((c - 1) * batteryType)); startRow = ((r * thicknessWall) + ((r - 1) * batteryType)); translate([startColumn, startRow, thicknessWall]) cube([batteryType, batteryType, heightCompartment + 1]); } Side note, related to declaring variables but not an issue that you encountered: OpenSCAD does not allow setting the value of a variable more than once. So this code is illegal: x = 1; echo(x); x = 2; echo(x); Second note is that whenever you introduce a construct such as {union(), difference(), if(), for()}, variables declared within the braces are scoped, so they don't exist between iterations or after the scope ends. if (true) { y = -1; } echo(y); // Illegal However, just introducing braces without a construct does not restrict the variable to the scope: { z = 5; } echo(z); // Legal