diff --git a/wasp/draw565.py b/wasp/draw565.py
index 4c5f25f..a94b812 100644
--- a/wasp/draw565.py
+++ b/wasp/draw565.py
@@ -2,7 +2,7 @@ import fonts.sans24
 import micropython
 
 @micropython.viper
-def bitblit(bitbuf, pixels, bgfg: int, count: int):
+def _bitblit(bitbuf, pixels, bgfg: int, count: int):
     mv = ptr8(bitbuf)
     px = ptr8(pixels)
 
@@ -29,7 +29,7 @@ def bitblit(bitbuf, pixels, bgfg: int, count: int):
             bitselect = 0x80
             pxp += 1
 
-def bounding_box(s, font):
+def _bounding_box(s, font):
     w = 0
     for ch in s:
         (_, h, wc) = font.get_ch(ch)
@@ -37,37 +37,63 @@ def bounding_box(s, font):
 
     return (w, h)
 
-def draw_glyph(display, glyph, x, y, bgfg):
+def _draw_glyph(display, glyph, x, y, bgfg):
     (px, h, w) = glyph
 
     buf = memoryview(display.linebuffer)[0:2*(w+1)]
     bytes_per_row = (w + 7) // 8
 
     for row in range(h):
-        bitblit(buf, px[row*bytes_per_row:], bgfg, w)
+        _bitblit(buf, px[row*bytes_per_row:], bgfg, w)
         buf[2*w] = 0
         buf[2*w + 1] = 0
         display.rawblit(buf, x, y+row, w+1, 1)
 
 class Draw565(object):
+    """Drawing library for RGB565 displays.
+
+    A full framebufer is not required although the library will
+    'borrow' a line buffer from the underlying display driver.
+    """
+
     def __init__(self, display):
+        """Initialise the library.
+
+        Defaults to white-on-black for monochrome drawing operations
+        and 24 pt Sans Serif text.
+        """
         self._display = display
         self.set_color(0xffff)
         self.set_font(fonts.sans24)
 
     def set_color(self, color, bg=0):
+        """Set the foreground (color) and background (bg) color.
+
+        The supplied color will be used for all monochrome drawing operations.
+        If no background color is provided then the background will be set
+        to black.
+        """
         self._bgfg = (bg << 16) + color
 
     def set_font(self, font):
+        """Set the font used for rendering text."""
         self._font = font
 
     def string(self, s, x, y, width=None):
+        """Draw a string at the supplied position.
+
+        If no width is provided then the text will be left justified,
+        otherwise the text will be centered within the provided width and,
+        importantly, the remaining width will be filled with the background
+        color (to ensure that if we update one string with a narrower one
+        there is no need to "undraw" it)
+        """
         display = self._display
         bgfg = self._bgfg
         font = self._font
 
         if width:
-            (w, h) = bounding_box(s, font)
+            (w, h) = _bounding_box(s, font)
             leftpad = (width - w) // 2
             rightpad = width - w - leftpad
             display.fill(0, x, y, leftpad, h)
@@ -75,17 +101,8 @@ class Draw565(object):
 
         for ch in s:
             glyph = font.get_ch(ch)
-            draw_glyph(display, glyph, x, y, bgfg)
+            _draw_glyph(display, glyph, x, y, bgfg)
             x += glyph[2] + 1
 
         if width:
             display.fill(0, x, y, rightpad, h)
-
-#import watch
-#draw = Draw(watch.display)
-#
-#def test():
-#    watch.display.poweron()
-#    watch.backlight.set(2)
-#
-#    draw.string('10-Jan-2020', 0, 24, width=240)