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diff --git a/doc/exact_match.txt b/doc/exact_match.txt
index d749552..2dbb277 100644
--- a/doc/exact_match.txt
+++ b/doc/exact_match.txt
@@ -27,3 +27,30 @@ on TitleAndResponsibility, you would use:
 <input type="text" name="v1">
 <input type="hidden" name="e1" value="5">
 
+
+What are bit-masks?
+
+Bit-mask is usage of one byte (8 bits) as 8 separate bits with it's own
+meaning (this is simplification, but bear with me for now).
+
+So, 1 = 2^0, thus it's bit 1. With analogy, 2=2^1 and 3=2^0+2^1. 
+So, for 1-3 we use two bits and have:
+
+number	bits
+1	01	(just begin bit set)
+2	10	(just end bit set)
+3	11	(begin and end bit set)
+
+Thus, with two bits (and values 1-3) we can express should we exact match from
+beginning, end or both. For wild-card match, we use additional bit 3 (2^3 = 4)
+so we have:
+
+number	bits	exact match
+1	001	begin
+2	010	end
+3	011	begin+end
+4	100	(not used)
+5	101	(4+1) begin+wild-card
+6	110	(4+2) end+wild-card
+7	111	(4+3) begin+end+wild-card
+