diff --git a/core/src/main/java/com/google/googlejavaformat/java/CommandLineOptions.java b/core/src/main/java/com/google/googlejavaformat/java/CommandLineOptions.java
index 5a233284..d5480a79 100644
--- a/core/src/main/java/com/google/googlejavaformat/java/CommandLineOptions.java
+++ b/core/src/main/java/com/google/googlejavaformat/java/CommandLineOptions.java
@@ -16,6 +16,8 @@
 
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableRangeSet;
+import com.google.common.collect.RangeSet;
+import com.google.common.collect.TreeRangeSet;
 import java.util.Optional;
 
 /**
@@ -178,7 +180,7 @@ static Builder builder() {
   static class Builder {
 
     private final ImmutableList.Builder<String> files = ImmutableList.builder();
-    private final ImmutableRangeSet.Builder<Integer> lines = ImmutableRangeSet.builder();
+    private final RangeSet<Integer> lines = TreeRangeSet.create();
     private final ImmutableList.Builder<Integer> offsets = ImmutableList.builder();
     private final ImmutableList.Builder<Integer> lengths = ImmutableList.builder();
     private boolean inPlace = false;
@@ -204,7 +206,7 @@ Builder inPlace(boolean inPlace) {
       return this;
     }
 
-    ImmutableRangeSet.Builder<Integer> linesBuilder() {
+    RangeSet<Integer> linesBuilder() {
       return lines;
     }
 
@@ -282,7 +284,7 @@ CommandLineOptions build() {
       return new CommandLineOptions(
           files.build(),
           inPlace,
-          lines.build(),
+          ImmutableRangeSet.copyOf(lines),
           offsets.build(),
           lengths.build(),
           aosp,
diff --git a/core/src/main/java/com/google/googlejavaformat/java/CommandLineOptionsParser.java b/core/src/main/java/com/google/googlejavaformat/java/CommandLineOptionsParser.java
index f7c3dec9..52a5e05d 100644
--- a/core/src/main/java/com/google/googlejavaformat/java/CommandLineOptionsParser.java
+++ b/core/src/main/java/com/google/googlejavaformat/java/CommandLineOptionsParser.java
@@ -18,8 +18,8 @@
 
 import com.google.common.base.CharMatcher;
 import com.google.common.base.Splitter;
-import com.google.common.collect.ImmutableRangeSet;
 import com.google.common.collect.Range;
+import com.google.common.collect.RangeSet;
 import java.io.IOException;
 import java.io.UncheckedIOException;
 import java.nio.file.Files;
@@ -157,7 +157,7 @@ private static String getValue(String flag, Iterator<String> it, String value) {
    * number. Line numbers are {@code 1}-based, but are converted to the {@code 0}-based numbering
    * used internally by google-java-format.
    */
-  private static void parseRangeSet(ImmutableRangeSet.Builder<Integer> result, String ranges) {
+  private static void parseRangeSet(RangeSet<Integer> result, String ranges) {
     for (String range : COMMA_SPLITTER.split(ranges)) {
       result.add(parseRange(range));
     }
diff --git a/core/src/main/java/com/google/googlejavaformat/java/UsageException.java b/core/src/main/java/com/google/googlejavaformat/java/UsageException.java
index a10f2d07..50d55d4d 100644
--- a/core/src/main/java/com/google/googlejavaformat/java/UsageException.java
+++ b/core/src/main/java/com/google/googlejavaformat/java/UsageException.java
@@ -56,7 +56,7 @@ final class UsageException extends Exception {
     "  --set-exit-if-changed",
     "    Return exit code 1 if there are any formatting changes.",
     "  --lines, -lines, --line, -line",
-    "    Line range(s) to format, like 5:10 (1-based; default is all).",
+    "    Line range(s) to format, e.g. the first 5 lines are 1:5 (1-based; default is all).",
     "  --offset, -offset",
     "    Character offset to format (0-based; default is all).",
     "  --length, -length",
diff --git a/core/src/test/java/com/google/googlejavaformat/java/CommandLineOptionsParserTest.java b/core/src/test/java/com/google/googlejavaformat/java/CommandLineOptionsParserTest.java
index 08fbbbab..93dfb79c 100644
--- a/core/src/test/java/com/google/googlejavaformat/java/CommandLineOptionsParserTest.java
+++ b/core/src/test/java/com/google/googlejavaformat/java/CommandLineOptionsParserTest.java
@@ -16,7 +16,6 @@
 
 import static com.google.common.truth.Truth.assertThat;
 import static java.nio.charset.StandardCharsets.UTF_8;
-import static org.junit.Assert.fail;
 
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Range;
@@ -151,15 +150,13 @@ public void setExitIfChanged() {
         .isTrue();
   }
 
-  // TODO(cushon): consider handling this in the parser and reporting a more detailed error
   @Test
-  public void illegalLines() {
-    try {
-      CommandLineOptionsParser.parse(Arrays.asList("-lines=1:1", "-lines=1:1"));
-      fail();
-    } catch (IllegalArgumentException e) {
-      assertThat(e).hasMessageThat().contains("overlap");
-    }
+  public void mergedLines() {
+    assertThat(
+            CommandLineOptionsParser.parse(Arrays.asList("-lines=1:5", "-lines=2:8"))
+                .lines()
+                .asRanges())
+        .containsExactly(Range.closedOpen(0, 8));
   }
 
   @Test