diff --git a/examples/rotating-ellipse.py b/examples/rotating-ellipse.py
index 6bf8e06..2482d58 100755
--- a/examples/rotating-ellipse.py
+++ b/examples/rotating-ellipse.py
@@ -1,11 +1,34 @@
 #!/usr/bin/env python
 
-# Create grids for a rotating ellipse stellarator, based on curvilinear grids
-
+import argparse
 import numpy as np
-
 import zoidberg
 
+parser = argparse.ArgumentParser(
+    prog="RotatingEllipse",
+    description="Create grids for a rotating ellipse stellarator, based on curvilinear grids.",
+)
+parser.add_argument(
+    "-nx", type=int, default=20, help="Number of radial (x) cells including boundaries"
+)
+parser.add_argument(
+    "-ny", "--nslices", type=int, default=8, help="Number of parallel (y) slices"
+)
+parser.add_argument("-nz", type=int, default=20, help="Number of poloidal (z) cells")
+parser.add_argument(
+    "-ns",
+    "--nsamples-per-dim",
+    type=int,
+    default=1,
+    help="Number of samples per dimension",
+)
+parser.add_argument("-pb", "--partial-boundaries", action="store_true", default=True)
+parser.add_argument("-o", "--output", type=str, default="")
+args = parser.parse_args()
+
+if args.output == "":
+    args.output = f"rotating-ellipse-{args.nx}x{args.nslices}x{args.nz}-s{args.nsamples_per_dim}{'-npb' if not args.partial_boundaries else ''}.fci.nc"
+
 #############################################################################
 # Define the magnetic field
 
@@ -30,8 +53,7 @@
 start_r = xcentre + 0.4
 start_z = 0.0
 
-nslices = 8  # Number of poloidal slices
-ycoords = np.linspace(0, yperiod, nslices)
+ycoords = np.linspace(0, yperiod, args.nslices, endpoint=False)
 npoints = 20  # Points per poloidal slice
 
 rzcoord, ycoords = zoidberg.fieldtracer.trace_poincare(
@@ -39,7 +61,7 @@
 )
 
 inner_lines = []
-for i in range(nslices):
+for i in range(args.nslices):
     r = rzcoord[:, i, 0, 0]
     z = rzcoord[:, i, 0, 1]
     line = zoidberg.rzline.line_from_points(r, z)
@@ -59,11 +81,8 @@
 # Now have inner and outer boundaries for each poloidal grid
 # Generate a grid on each poloidal slice using the elliptic grid generator
 
-nx = 20
-nz = 20
-
 pol_grids = [
-    zoidberg.poloidal_grid.grid_elliptic(inner_line, outer_line, nx, nz)
+    zoidberg.poloidal_grid.grid_elliptic(inner_line, outer_line, args.nx, args.nz)
     for inner_line in inner_lines
 ]
 
@@ -72,19 +91,14 @@
 
 grid = zoidberg.grid.Grid(pol_grids, ycoords, yperiod, yperiodic=True)
 
-samples_per_dim = 2  # Sub-sampling in each cell
-partial_boundaries = True
+maps = zoidberg.make_maps(grid, magnetic_field, samples_per_dim=args.nsamples_per_dim)
 
-maps = zoidberg.make_maps(grid, magnetic_field, samples_per_dim=samples_per_dim)
-
-maps = zoidberg.weights.modify_maps(maps, partial_boundaries=partial_boundaries)
+maps = zoidberg.weights.modify_maps(maps, partial_boundaries=args.partial_boundaries)
 
 #############################################################################
 # Write grid file
 
-filename = f"rotating-ellipse-{nx}x{nslices}x{nz}-s{samples_per_dim}{'-npb' if not partial_boundaries else ''}.fci.nc"
-
-print(f"Writing to grid file '{filename}'")
+print(f"Writing to grid file '{args.output}'")
 zoidberg.write_maps(
-    grid, magnetic_field, maps, gridfile=filename, new_names=False, metric2d=False
+    grid, magnetic_field, maps, gridfile=args.output, new_names=False, metric2d=False
 )
diff --git a/zoidberg/grid.py b/zoidberg/grid.py
index ea384eb..947f612 100644
--- a/zoidberg/grid.py
+++ b/zoidberg/grid.py
@@ -49,9 +49,27 @@ def __init__(self, poloidal_grids, ycoords, Ly, yperiodic=False, name="fci_grid"
 
         self.poloidal_grids = poloidal_grids
         self.ycoords = np.asarray(ycoords)
+
+        if len(self.ycoords.shape) != 1:
+            raise ValueError("ycoords must be 1-dimensional")
+
+        def is_monotonic(a):
+            return np.all(a[1:] >= a[:-1])
+
+        if not is_monotonic(self.ycoords):
+            raise ValueError("ycoords must be monotonically increasing")
+
         self.Ly = Ly
         self.yperiodic = yperiodic
 
+        if yperiodic:
+            if (
+                Ly <= self.ycoords[-1] - self.ycoords[0] + 1e-10
+            ):  # Note: Catch case where they are equal
+                raise ValueError(
+                    f"Ly={Ly} must be > max(ycoords) - min(ycoords) = {ycoords[-1]} - {ycoords[0]}"
+                )
+
         self._metric_cache = None
 
         # Define the shape of the grid