diff --git a/indicators/percent-rank/README.md b/indicators/percent-rank/README.md
index b504398..d9fd2e6 100644
--- a/indicators/percent-rank/README.md
+++ b/indicators/percent-rank/README.md
@@ -4,4 +4,8 @@ The Percent Rank indicator calculates the percentage of input values which are l
 
 By default it operates on closing prices, but it can be applied to any input series.
 
+## Parameters
+
+**Period**: The period over which to calculate the percent rank. (Default: 100)
+
 ---
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