diff --git a/src/Common/DenseHashMap.h b/src/Common/DenseHashMap.h
index 26b9224528b..9ac21c82676 100644
--- a/src/Common/DenseHashMap.h
+++ b/src/Common/DenseHashMap.h
@@ -1,6 +1,11 @@
 #pragma once
 #include <unordered_map>
 
+/// DenseHashMap is a wrapper for google::dense_hash_map.
+/// Some hacks are needed to make it work in "Arcadia".
+/// "Arcadia" is a proprietary monorepository in Yandex.
+/// It uses slightly changed version of sparsehash with a different set of hash functions (which we don't need).
+/// Those defines are needed to make it compile.
 #if defined(ARCADIA_BUILD)
 #define HASH_FUN_H <unordered_map>
 template <typename T>
diff --git a/src/Common/DenseHashSet.h b/src/Common/DenseHashSet.h
index 5b30d88a70c..e8c06f36aa3 100644
--- a/src/Common/DenseHashSet.h
+++ b/src/Common/DenseHashSet.h
@@ -1,5 +1,7 @@
 #pragma once
 
+/// DenseHashSet is a wrapper for google::dense_hash_set.
+/// See comment in DenseHashMap.h
 #if defined(ARCADIA_BUILD)
 #define HASH_FUN_H <unordered_map>
 template <typename T>
diff --git a/src/Common/SparseHashMap.h b/src/Common/SparseHashMap.h
index 403042c11a9..f01fc633d84 100644
--- a/src/Common/SparseHashMap.h
+++ b/src/Common/SparseHashMap.h
@@ -1,5 +1,7 @@
 #pragma once
 
+/// SparseHashMap is a wrapper for google::sparse_hash_map.
+/// See comment in DenseHashMap.h
 #if defined(ARCADIA_BUILD)
 #define HASH_FUN_H <unordered_map>
 template <typename T>