diff --git a/src/sdr/_iir.py b/src/sdr/_iir.py
index 1ceaa9e1f..6dac47fad 100644
--- a/src/sdr/_iir.py
+++ b/src/sdr/_iir.py
@@ -1,3 +1,6 @@
+"""
+A module for infinite impulse response (IIR) filters.
+"""
 import matplotlib.pyplot as plt
 import numpy as np
 import scipy.signal