diff --git a/src/_impl/_contours_impl.scad b/src/_impl/_contours_impl.scad
index 92c19ab4..c266c6d7 100644
--- a/src/_impl/_contours_impl.scad
+++ b/src/_impl/_contours_impl.scad
@@ -21,7 +21,7 @@ function interpolated_pt(p0, p1, threshold) =
 function _isolines_pn_label(pts, threshold) =
     [
         for(row = pts)
-        [for(p = row) [p[0], p[1], p[2], p[2] - threshold >= 0]]
+        [for(p = row) [p[0], p[1], p[2], p[2] >= threshold]]
     ]; 
     
 function _isolines_corner_value(cell_pts) =
@@ -587,7 +587,7 @@ function _case2020_isobands(cell_pts, lower, upper) =
             interpolated_pt(cell_pts[3], center_p, lower)
         ]
     ] :
-    center_p_z >= lower && center_p_z < upper ? [
+    center_p_z >= lower && center_p_z <= upper ? [
         [
             interpolated_pt(cell_pts[0], cell_pts[1], lower),
             interpolated_pt(cell_pts[0], center_p, lower),
@@ -644,7 +644,7 @@ function _case0202_isobands(cell_pts, lower, upper) =
             interpolated_pt(cell_pts[1], cell_pts[2], upper)
         ]        
     ] :
-    center_p_z >= lower && center_p_z < upper ? [
+    center_p_z >= lower && center_p_z <= upper ? [
         [
             interpolated_pt(cell_pts[0], cell_pts[1], upper),
             interpolated_pt(cell_pts[0], center_p, upper),
@@ -697,7 +697,7 @@ function _case0101_isobands(cell_pts, lower, upper) =
             interpolated_pt(cell_pts[1], cell_pts[2], lower)
         ]        
     ] :
-    center_p_z >= lower && center_p_z < upper ? [
+    center_p_z >= lower && center_p_z <= upper ? [
         [
             [cell_pts[0][0], cell_pts[0][1], cell_pts[0][2]],
             interpolated_pt(cell_pts[0], cell_pts[3], lower),
@@ -750,7 +750,7 @@ function _case1010_isobands(cell_pts, lower, upper) =
             interpolated_pt(cell_pts[2], cell_pts[3], lower)
         ]        
     ] : 
-    center_p_z >= lower && center_p_z < upper ? [
+    center_p_z >= lower && center_p_z <= upper ? [
         [
             interpolated_pt(cell_pts[0], center_p, lower),
             interpolated_pt(cell_pts[0], cell_pts[3], lower),
@@ -784,30 +784,56 @@ function _case1010_isobands(cell_pts, lower, upper) =
         ]
     ];    
 
-function _case2121_isobands(cell_pts, threshold) = 
+function _case2121_isobands(cell_pts, lower, upper) = 
     let(
         center_p = (cell_pts[0] + cell_pts[1] + cell_pts[2] + cell_pts[3]) / 4,
         center_p_z = center_p[2]
     )
-    center_p_z < threshold ? [
+    center_p_z < lower ? [
+        [
+            interpolated_pt(cell_pts[1], cell_pts[2], upper),
+            interpolated_pt(cell_pts[1], center_p, upper),
+            interpolated_pt(cell_pts[1], center_p, lower),
+            interpolated_pt(cell_pts[2], center_p, lower),
+            interpolated_pt(cell_pts[3], center_p, lower),
+            interpolated_pt(cell_pts[3], center_p, upper),
+            interpolated_pt(cell_pts[2], cell_pts[3], upper),
+            [cell_pts[2][0], cell_pts[2][1], cell_pts[2][2]]
+        ],
+        [
+            interpolated_pt(cell_pts[1], center_p, upper),
+            interpolated_pt(cell_pts[0], cell_pts[1], upper),
+            [cell_pts[0][0], cell_pts[0][1], cell_pts[0][2]],
+            interpolated_pt(cell_pts[0], cell_pts[3], upper),
+            interpolated_pt(cell_pts[3], center_p, upper),
+            interpolated_pt(cell_pts[3], center_p, lower),
+            interpolated_pt(cell_pts[0], center_p, lower),
+            interpolated_pt(cell_pts[1], center_p, lower)
+        ]
+    ] :
+    center_p_z >= lower && center_p_z <= upper ? [
         [
             [cell_pts[0][0], cell_pts[0][1], cell_pts[0][2]],
-            interpolated_pt(cell_pts[0], cell_pts[3], threshold),
-            interpolated_pt(cell_pts[2], cell_pts[3], threshold),
+            interpolated_pt(cell_pts[0], cell_pts[3], upper),
+            interpolated_pt(cell_pts[0], center_p, upper),
+            interpolated_pt(cell_pts[2], cell_pts[3], upper),
             [cell_pts[2][0], cell_pts[2][1], cell_pts[2][2]],
-            interpolated_pt(cell_pts[1], cell_pts[2], threshold),
-            interpolated_pt(cell_pts[0], cell_pts[1], threshold)
+            interpolated_pt(cell_pts[1], cell_pts[2], upper),
+            interpolated_pt(cell_pts[1], center_p, upper),
+            interpolated_pt(cell_pts[0], cell_pts[1], upper)
         ]
     ] : [
         [
             [cell_pts[0][0], cell_pts[0][1], cell_pts[0][2]],
-            interpolated_pt(cell_pts[0], cell_pts[3], threshold),
-            interpolated_pt(cell_pts[0], cell_pts[1], threshold)
+            interpolated_pt(cell_pts[0], cell_pts[3], upper),
+            interpolated_pt(cell_pts[0], center_p, upper),
+            interpolated_pt(cell_pts[0], cell_pts[1], upper)
         ],
         [
-            interpolated_pt(cell_pts[2], cell_pts[3], threshold),
+            interpolated_pt(cell_pts[2], cell_pts[3], upper),
             [cell_pts[2][0], cell_pts[2][1], cell_pts[2][2]],
-            interpolated_pt(cell_pts[1], cell_pts[2], threshold)
+            interpolated_pt(cell_pts[1], cell_pts[2], upper),
+            interpolated_pt(cell_pts[2], center_p, upper)
         ]        
     ];        
 
@@ -1150,7 +1176,7 @@ function _isobands_of(cell_pts, lower, upper) =
     cv == "0202" ? _case0202_isobands(cell_pts, lower, upper) :
     cv == "0101" ? _case0101_isobands(cell_pts, lower, upper) :
     cv == "1010" ? _case1010_isobands(cell_pts, lower, upper) :
-    cv == "2121" ? _case2121_isobands(cell_pts, upper) :    
+    cv == "2121" ? _case2121_isobands(cell_pts, lower, upper) :    
     cv == "1212" ? _case1212_isobands(cell_pts, upper) : 
     cv == "2120" ? _case2120_isobands(cell_pts, lower, upper) :    
     cv == "0102" ? _case0102_isobands(cell_pts, lower, upper) :