client.go                               | 1 +
 connection.go                           | 4 +++-
 internal/pieceordering/pieceordering.go | 7 ++++++-
 piece.go                                | 6 ++++--

diff --git a/client.go b/client.go
index 27af8fc705d8e1b60b8e95c08e990985bfe97411..ee15ee0fb7164fe31f14d1f904ba3408e3009f72 100644
--- a/client.go
+++ b/client.go
@@ -1975,6 +1975,7 @@ 	}
 	return
 }
 
+// Marks the pieces in the given region for download.
 func (t Torrent) SetRegionPriority(off, len int64) {
 	t.cl.mu.Lock()
 	defer t.cl.mu.Unlock()
diff --git a/connection.go b/connection.go
index ed94626ff3a890b1b0d1242045374fa0708f548c..1bf69b15f7b6c9b68fafe459778ffe75b6d1fa13 100644
--- a/connection.go
+++ b/connection.go
@@ -39,7 +39,8 @@ 	mu        sync.Mutex // Only for closing.
 	post      chan pp.Message
 	writeCh   chan []byte
 
-	// The connections preferred order to download pieces.
+	// The connection's preferred order to download pieces. The index is the
+	// piece, the value is its priority.
 	piecePriorities []int
 	// The piece request order based on piece priorities.
 	pieceRequestOrder *pieceordering.Instance
@@ -109,6 +110,7 @@ 	pp := cn.piecePriorities[piece]
 	// Priority regions not to scale. Within each region, piece is randomized
 	// according to connection.
 
+	// <-request first -- last->
 	// [ Now         ]
 	//  [ Next       ]
 	//   [ Readahead ]
diff --git a/internal/pieceordering/pieceordering.go b/internal/pieceordering/pieceordering.go
index d26cbdcd2d1b55ae09254be760e74fc1d08e48a8..169ebbc7ee2ef50356c7d679ff14ffdb5fa0690f 100644
--- a/internal/pieceordering/pieceordering.go
+++ b/internal/pieceordering/pieceordering.go
@@ -10,7 +10,11 @@ )
 
 // Maintains piece integers by their ascending assigned keys.
 type Instance struct {
-	sl        *skiplist.SkipList
+	// Contains the ascending priority keys. The keys contain a slice of piece
+	// indices.
+	sl *skiplist.SkipList
+	// Maps from piece index back to its key, so that it can be remove
+	// efficiently from the skip list.
 	pieceKeys map[int]int
 }
 
@@ -41,6 +45,7 @@ 	me.pieceKeys[piece] = key
 	me.shuffleItem(key)
 }
 
+// Shuffle the piece indices that share a given key.
 func (me *Instance) shuffleItem(key int) {
 	_item, ok := me.sl.Get(key)
 	if !ok {
diff --git a/piece.go b/piece.go
index 6bb96a3e69b428cf9b2535ec881b0e4ceef43d76..c45e7a423bedbe99b318362b268b675ea0faaeb2 100644
--- a/piece.go
+++ b/piece.go
@@ -7,6 +7,8 @@
 	pp "github.com/anacrolix/torrent/peer_protocol"
 )
 
+// Piece priority describes the importance of obtaining a particular piece.
+
 type piecePriority byte
 
 const (
@@ -18,8 +20,8 @@ 	piecePriorityNow
 )
 
 type piece struct {
-	Hash pieceSum
-	// Chunks we don't have. The offset and length can be determined by our
+	Hash pieceSum // The completed piece SHA1 hash, from the metainfo "pieces" field.
+	// Chunks we don't have. The offset and length can be determined by the
 	// request chunkSize in use.
 	PendingChunkSpecs []bool
 	Hashing           bool