diff --git a/src/sam/reader.jl b/src/sam/reader.jl
index 62a6ad5..46fd286 100644
--- a/src/sam/reader.jl
+++ b/src/sam/reader.jl
@@ -12,7 +12,7 @@ function Reader(state::State{S}) where {S <: TranscodingStream}
 
     cs, ln = readheader!(rdr.state.stream, rdr.header, (sam_machine_header.start_state, rdr.state.linenum))
 
-    rdr.state.state = sam_machine_body.start_state
+    rdr.state.state = sam_machine_body.start_state # Get the reader ready to read the body.
     rdr.state.linenum = ln
     rdr.state.filled = false
 
diff --git a/src/sam/record.jl b/src/sam/record.jl
index dcddf42..7dde515 100644
--- a/src/sam/record.jl
+++ b/src/sam/record.jl
@@ -2,10 +2,11 @@
 # ==========
 
 mutable struct Record
-    # data and filled range
+    # Data and filled range.
     data::Vector{UInt8}
-    filled::UnitRange{Int}
-    # indexes
+    filled::UnitRange{Int} # Note: Specifies the data in use.
+
+    # Mandatory fields.
     qname::UnitRange{Int}
     flag::UnitRange{Int}
     rname::UnitRange{Int}
@@ -17,6 +18,8 @@ mutable struct Record
     tlen::UnitRange{Int}
     seq::UnitRange{Int}
     qual::UnitRange{Int}
+
+    # Auxiliary fields.
     fields::Vector{UnitRange{Int}}
 end