diff --git a/code/api/index-api/src/main/protobuf/index-api.proto b/code/api/index-api/src/main/protobuf/index-api.proto
index bc177746..6862857f 100644
--- a/code/api/index-api/src/main/protobuf/index-api.proto
+++ b/code/api/index-api/src/main/protobuf/index-api.proto
@@ -13,6 +13,7 @@ service IndexApi {
 
 message Empty {}
 
+/* Query Service query request */
 message RpcQsQuery {
   string humanQuery = 1;
   string nearDomain = 2;
@@ -29,6 +30,7 @@ message RpcQsQuery {
   string searchSetIdentifier = 13;
 }
 
+/* Query service query response */
 message RpcQsResponse {
     RpcIndexQuery specs = 1;
     repeated RpcDecoratedResultItem results = 2;
@@ -37,20 +39,22 @@ message RpcQsResponse {
     string domain = 5;
 }
 
+/* Index service query request */
 message RpcIndexQuery {
   repeated RpcSubquery subqueries = 1;
-  repeated int32 domains = 2;
-  string searchSetIdentifier = 3;
-  string humanQuery = 4;
+  repeated int32 domains = 2;     // (optional) A list of domain IDs to consider
+  string searchSetIdentifier = 3; // (optional) A named set of domains to consider
+  string humanQuery = 4;          // The search query as the user entered it
   RpcSpecLimit quality = 5;
   RpcSpecLimit year = 6;
   RpcSpecLimit size = 7;
   RpcSpecLimit rank = 8;
   RpcQueryLimits queryLimits = 9;
-  string queryStrategy = 10;
+  string queryStrategy = 10;      // Named query configuration
   RpcResultRankingParameters parameters = 11;
 }
 
+/* A tagged union encoding some limit on a field */
 message RpcSpecLimit {
   int32 value = 1;
   TYPE type = 2;
@@ -63,10 +67,7 @@ message RpcSpecLimit {
   };
 }
 
-message RpcSearchResultSet {
-  repeated RpcDecoratedResultItem items = 1;
-}
-
+/** A search result item decorated with title and description metadata from the link database */
 message RpcDecoratedResultItem {
   RpcRawResultItem rawItem = 1;
   string url = 2;
@@ -74,40 +75,43 @@ message RpcDecoratedResultItem {
   string description = 4;
   double urlQuality = 5;
   string format = 6;
-  int32 features = 7;
+  int32 features = 7; // bitmask encoding features of the document
   int32 pubYear = 8;
   int64 dataHash = 9;
   int32 wordsTotal = 10;
-  double rankingScore = 11;
+  double rankingScore = 11; // The ranking score of this search result item, lower is better
 }
 
+/** A raw index-service view of a search result */
 message RpcRawResultItem {
-  int64 combinedId = 1;
-  int32 resultsFromDomain = 2;
+  int64 combinedId = 1; // raw ID with bit-encoded ranking information still present
+  int32 resultsFromDomain = 2; // number of other results from the same domain
   repeated RpcResultKeywordScore keywordScores = 3;
 }
 
+/* Information about how well a keyword matches a query */
 message RpcResultKeywordScore {
-  int32 subquery = 1;
-  string keyword = 2;
-  int64 encodedWordMetadata = 3;
-  int64 encodedDocMetadata = 4;
-  bool hasPriorityTerms = 5;
-  int32 htmlFeatures = 6;
+  int32 subquery = 1; // index of the subquery this keyword relates to
+  string keyword = 2; // the keyword
+  int64 encodedWordMetadata = 3; // bit encoded word metadata
+  int64 encodedDocMetadata = 4; // bit encoded document metadata
+  bool hasPriorityTerms = 5; // true if this word is important to the document
+  int32 htmlFeatures = 6; // bit encoded document features
 }
 
+/* Query execution parameters */
 message RpcQueryLimits {
   int32 resultsByDomain = 1;
   int32 resultsTotal = 2;
   int32 timeoutMs = 3;
-  int32 fetchSize = 4;
+  int32 fetchSize = 4; // Size of the fetch buffer in the index service
 }
 
 message RpcResultRankingParameters {
-  double fullK = 1;
-  double fullB = 2;
-  double prioK = 3;
-  double prioB = 4;
+  double fullK = 1; // BM25 parameter
+  double fullB = 2; // BM25 parameter
+  double prioK = 3; // BM25 parameter
+  double prioB = 4; // BM25 parameter
   int32 shortDocumentThreshold = 5;
   double shortDocumentPenalty = 6;
   double domainRankBonus = 7;
@@ -122,18 +126,21 @@ message RpcResultRankingParameters {
 
   enum TEMPORAL_BIAS {
       NONE = 0;
-      RECENT = 1;
-      OLD = 2;
+      RECENT = 1; // Prefer recent documents
+      OLD = 2; // Prefer older documents
   }
 }
+
+/* Defines a single subquery */
 message RpcSubquery {
-  repeated string include = 1;
-  repeated string exclude = 2;
-  repeated string advice = 3;
-  repeated string priority = 4;
-  repeated RpcCoherences coherences = 5;
+  repeated string include = 1;  // These terms must be present
+  repeated string exclude = 2;  // These terms must be absent
+  repeated string advice = 3;   // These terms must be present, but do not affect ranking
+  repeated string priority = 4;  // These terms are not mandatory, but affect ranking positively if they are present
+  repeated RpcCoherences coherences = 5; // Groups of terms that must exist in proximity of each other
 }
 
+/* Defines a group of search terms that must exist in close proximity within the document */
 message RpcCoherences {
   repeated string coherences = 1;
 }