diff --git a/include/osmocore/bits.h b/include/osmocore/bits.h
index eb2cbcb..8d4a078 100644
--- a/include/osmocore/bits.h
+++ b/include/osmocore/bits.h
@@ -7,6 +7,12 @@
 typedef uint8_t ubit_t;		/* unpacked bit (0 or 1) */
 typedef uint8_t pbit_t;		/* packed bis (8 bits in a byte) */
 
+/*
+   NOTE on the endianess of pbit_t:
+   Bits in a pbit_t are ordered MSB first, i.e. 0x80 is the first bit.
+   Bit i in a pbit_t array is array[i/8] & (1<<(7-i%8))
+*/
+
 /* determine how many bytes we would need for 'num_bits' packed bits */
 static inline unsigned int osmo_pbit_bytesize(unsigned int num_bits)
 {