diff --git a/fgpyo/sam/__init__.py b/fgpyo/sam/__init__.py
index 1d80beb..a8d465f 100644
--- a/fgpyo/sam/__init__.py
+++ b/fgpyo/sam/__init__.py
@@ -677,33 +677,48 @@ def from_recs(  # noqa: C901  # `from_recs` is too complex (11 > 10)
             return PairOrientation.RF
 
 
-def isize(r1: AlignedSegment, r2: Optional[AlignedSegment] = None) -> int:
-    """Computes the insert size for a pair of records."""
-    if r2 is None:
-        r2_is_unmapped = r1.mate_is_unmapped
-        r2_reference_id = r1.next_reference_id
+def isize(rec1: AlignedSegment, rec2: Optional[AlignedSegment] = None) -> int:
+    """Computes the insert size ("template length" or "TLEN") for a pair of records.
+
+    Args:
+        rec1: The first record in the pair.
+        rec2: The second record in the pair. If None, then mate info on `rec1` will be used.
+
+    """
+    if rec2 is None:
+        rec2_is_unmapped = rec1.mate_is_unmapped
+        rec2_reference_id = rec1.next_reference_id
     else:
-        r2_is_unmapped = r2.is_unmapped
-        r2_reference_id = r2.reference_id
+        rec2_is_unmapped = rec2.is_unmapped
+        rec2_reference_id = rec2.reference_id
 
-    if r1.is_unmapped or r2_is_unmapped or r1.reference_id != r2_reference_id:
+    if rec1.is_unmapped or rec2_is_unmapped or rec1.reference_id != rec2_reference_id:
         return 0
 
-    if r2 is None:
-        if not r1.has_tag("MC"):
-            raise ValueError('Cannot determine proper pair status without R2\'s cigar ("MC")!')
-        r2_cigar = Cigar.from_cigarstring(str(r1.get_tag("MC")))
-        r2_is_reverse = r1.mate_is_reverse
-        r2_reference_start = r1.next_reference_start
-        r2_reference_end = r1.next_reference_start + r2_cigar.length_on_target()
+    if rec2 is None:
+        rec2_is_forward = rec1.mate_is_forward
+        rec2_reference_start = rec1.next_reference_start
     else:
-        r2_is_reverse = r2.is_reverse
-        r2_reference_start = r2.reference_start
-        r2_reference_end = r2.reference_end
+        rec2_is_forward = rec2.is_forward
+        rec2_reference_start = rec2.reference_start
 
-    r1_pos = r1.reference_end if r1.is_reverse else r1.reference_start
-    r2_pos = r2_reference_end if r2_is_reverse else r2_reference_start
-    return r2_pos - r1_pos
+    if rec1.is_forward and rec2_is_forward:
+        return rec2_reference_start - rec1.reference_start
+    if rec1.is_reverse and rec2_is_forward:
+        return rec2_reference_start - rec1.reference_end
+
+    if rec2 is None:
+        if not rec1.has_tag("MC"):
+            raise ValueError('Cannot determine proper pair status without a mate cigar ("MC")!')
+        rec2_cigar = Cigar.from_cigarstring(str(rec1.get_tag("MC")))
+        rec2_reference_end = rec1.next_reference_start + rec2_cigar.length_on_target()
+    else:
+        rec2_reference_end = rec2.reference_end
+
+    if rec1.is_forward:
+        return rec2_reference_end - rec1.reference_start
+    else:
+        return rec2_reference_end - rec1.reference_end
 
 
 DefaultProperlyPairedOrientations: set[PairOrientation] = {PairOrientation.FR}
diff --git a/tests/fgpyo/sam/test_sam.py b/tests/fgpyo/sam/test_sam.py
index acbba6c..b5b6fc9 100755
--- a/tests/fgpyo/sam/test_sam.py
+++ b/tests/fgpyo/sam/test_sam.py
@@ -594,7 +594,9 @@ def test_isize_raises_when_r2_not_provided_and_no_mate_cigar_tag() -> None:
     builder = SamBuilder()
     r1, _ = builder.add_pair(chrom="chr1", start1=100, cigar1="115M", start2=250, cigar2="40M")
     r1.set_tag("MC", None)
-    with pytest.raises(ValueError, match="Cannot determine proper pair status without R2's cigar"):
+    with pytest.raises(
+        ValueError, match="Cannot determine proper pair status without a mate cigar"
+    ):
         sam.isize(r1)
 
 
@@ -769,7 +771,7 @@ def test_set_mate_info_one_unmapped() -> None:
 def test_set_mate_info_both_mapped() -> None:
     """Test set_mate_info sets mate info for two mapped records."""
     builder = SamBuilder()
-    r1, r2 = builder.add_pair(chrom="chr1", start1=200, start2=300)
+    r1, r2 = builder.add_pair(chrom="chr1", start1=200, start2=301)
     assert r1.is_mapped is True
     assert r2.is_mapped is True
 
@@ -789,11 +791,11 @@ def test_set_mate_info_both_mapped() -> None:
         assert rec.is_proper_pair is True
 
     assert r1.reference_start == 200
-    assert r1.next_reference_start == 300
-    assert r2.reference_start == 300
+    assert r1.next_reference_start == 301
+    assert r2.reference_start == 301
     assert r2.next_reference_start == 200
-    assert r1.template_length == 200
-    assert r2.template_length == -200
+    assert r1.template_length == 201
+    assert r2.template_length == -201
     assert r1.is_forward is True
     assert r2.is_reverse is True
     assert r1.mate_is_reverse is True