diff --git a/corrscope/triggers.py b/corrscope/triggers.py
index f63eaf5..08d5e2a 100644
--- a/corrscope/triggers.py
+++ b/corrscope/triggers.py
@@ -49,13 +49,6 @@ class MainTriggerConfig(
     post_trigger: Optional["PostTriggerConfig"] = None
     post_radius: Optional[int] = with_units("smp", default=3)
 
-    @property
-    def post_nsamp(self) -> Optional[int]:
-        if self.post_radius is not None:
-            return 2 * self.post_radius + 1
-        else:
-            return None
-
     def __attrs_post_init__(self):
         if self.edge_direction not in [-1, 1]:
             raise CorrError(f"{obj_name(self)}.edge_direction must be {{-1, 1}}")
@@ -74,9 +67,7 @@ class PostTriggerConfig(_TriggerConfig, KeywordAttrs):
     pass
 
 
-def register_trigger(
-    config_t: Type[_TriggerConfig]
-) -> "Callable[[Type[_Trigger]], Type[_Trigger]]":  # my god mypy-strict sucks
+def register_trigger(config_t: Type[_TriggerConfig]):
     """ @register_trigger(FooTriggerConfig)
     def FooTrigger(): ...
     """
@@ -136,7 +127,7 @@ class MainTrigger(_Trigger, ABC):
         if cfg.post_trigger:
             # Create a post-processing trigger, with narrow nsamp and stride=1.
             # This improves speed and precision.
-            self.post = cfg.post_trigger(self._wave, cfg.post_nsamp, 1, self._fps)
+            self.post = cfg.post_trigger(self._wave, cfg.post_radius, 1, self._fps)
         else:
             self.post = None
 
@@ -393,6 +384,15 @@ class CorrelationTriggerConfig(MainTriggerConfig, always_dump="pitch_tracking"):
 
 @register_trigger(CorrelationTriggerConfig)
 class CorrelationTrigger(MainTrigger):
+    """
+    Assume that if get_trigger(x) == x, then data[[x-1, x]] == [<0, >0].
+    - edge detectors [halfN = N//2] > 0.
+    - So wave.get_around(x)[N//2] > 0.
+    - So wave.get_around(x) = [x - N//2 : ...]
+
+    test_trigger() checks that get_around() works properly, for even/odd N.
+    """
+
     cfg: CorrelationTriggerConfig
 
     @property
@@ -793,7 +793,7 @@ class ZeroCrossingTrigger(PostTrigger):
 
     def get_trigger(self, index: int, cache: "PerFrameCache") -> int:
         # 'cache' is unused.
-        tsamp = self._tsamp
+        radius = self._tsamp
 
         if not 0 <= index < self._wave.nsamp:
             return index
@@ -809,7 +809,7 @@ class ZeroCrossingTrigger(PostTrigger):
         else:  # self._wave[sample] == 0
             return index + 1
 
-        data = self._wave[index : index + (direction * tsamp) : direction]
+        data = self._wave[index : index + direction * (radius + 1) : direction]
         # TODO remove unnecessary complexity, since diameter is probably under 10.
         intercepts = find(data, test)
         try:
@@ -817,7 +817,7 @@ class ZeroCrossingTrigger(PostTrigger):
             return index + (delta * direction) + int(value <= 0)
 
         except StopIteration:  # No zero-intercepts
-            return index + (direction * tsamp)
+            return index + (direction * radius)
 
         # noinspection PyUnreachableCode
         """
diff --git a/corrscope/wave.py b/corrscope/wave.py
index 4da1014..63b01f1 100644
--- a/corrscope/wave.py
+++ b/corrscope/wave.py
@@ -203,8 +203,8 @@ class Wave:
 
         Copies self.data[...] """
         distance = return_nsamp * stride
-        end = sample + distance // 2
-        begin = end - distance
+        begin = sample - distance // 2
+        end = begin + distance
         return self._get(begin, end, stride)
 
     def get_s(self) -> float:
diff --git a/tests/impulse24000.wav b/tests/impulse24000.wav
deleted file mode 100644
index 3fa2fbd..0000000
Binary files a/tests/impulse24000.wav and /dev/null differ
diff --git a/tests/test_trigger.py b/tests/test_trigger.py
index 8a07218..9e43b99 100644
--- a/tests/test_trigger.py
+++ b/tests/test_trigger.py
@@ -39,19 +39,28 @@ def cfg(trigger_diameter, pitch_tracking):
     )
 
 
-# I regret adding the nsamp_frame parameter. It makes unit tests hard.
-
 FPS = 60
 
+is_odd = parametrize("is_odd", [False, True])
 
-def test_trigger(cfg: CorrelationTriggerConfig):
-    wave = Wave("tests/impulse24000.wav")
+
+# CorrelationTrigger overall tests
+
+
+@is_odd
+@parametrize("post_trigger", [None, ZeroCrossingTriggerConfig()])
+def test_trigger(cfg: CorrelationTriggerConfig, is_odd: bool, post_trigger):
+    """Ensures that trigger can locate
+    the first positive sample of a -+ step exactly,
+    without off-by-1 errors."""
+    wave = Wave("tests/step2400.wav")
+    cfg = attr.evolve(cfg, post_trigger=post_trigger)
 
     iters = 5
     plot = False
-    x0 = 24000
-    x = x0 - 500
-    trigger: CorrelationTrigger = cfg(wave, 4000, stride=1, fps=FPS)
+    x0 = 2400
+    x = x0 - 50
+    trigger: CorrelationTrigger = cfg(wave, 400 + int(is_odd), stride=1, fps=FPS)
 
     if plot:
         BIG = 0.95
@@ -78,6 +87,10 @@ def test_trigger(cfg: CorrelationTriggerConfig):
 
 @parametrize("post_trigger", [None, ZeroCrossingTriggerConfig()])
 def test_post_stride(post_trigger):
+    """
+    Test that stride is respected when post_trigger is disabled,
+    and ignored when post_trigger is enabled.
+    """
     cfg = cfg_template(post_trigger=post_trigger)
 
     wave = Wave("tests/sine440.wav")
@@ -127,7 +140,9 @@ def test_trigger_direction(post_trigger, double_negate):
         assert trigger.get_trigger(index + dx, cache) == index
 
 
-def test_trigger_stride_edges(cfg: CorrelationTriggerConfig):
+def test_trigger_out_of_bounds(cfg: CorrelationTriggerConfig):
+    """Ensure out-of-bounds triggering with stride does not crash.
+    (why does stride matter? IDK.)"""
     wave = Wave("tests/sine440.wav")
     # period = 48000 / 440 = 109.(09)*
 
@@ -141,7 +156,7 @@ def test_trigger_stride_edges(cfg: CorrelationTriggerConfig):
     trigger.get_trigger(50000, PerFrameCache())
 
 
-def test_trigger_should_recalc_window():
+def test_when_does_trigger_recalc_window():
     cfg = cfg_template(recalc_semitones=1.0)
     wave = Wave("tests/sine440.wav")
     trigger: CorrelationTrigger = cfg(wave, tsamp=1000, stride=1, fps=FPS)
@@ -164,7 +179,34 @@ def test_trigger_should_recalc_window():
         assert trigger._is_window_invalid(x), x
 
 
-# Test pitch-invariant triggering using spectrum
+# Test post triggering by itself
+
+
+def test_post_trigger_radius():
+    """
+    Ensure ZeroCrossingTrigger has no off-by-1 errors when locating edges,
+    and slides at a fixed rate if no edge is found.
+    """
+    wave = Wave("tests/step2400.wav")
+    center = 2400
+    radius = 5
+
+    cfg = ZeroCrossingTriggerConfig()
+    post = cfg(wave, radius, 1, FPS)
+
+    cache = PerFrameCache(mean=0)
+
+    for offset in range(-radius, radius + 1):
+        assert post.get_trigger(center + offset, cache) == center, offset
+
+    for offset in [radius + 1, radius + 2, 100]:
+        assert post.get_trigger(center - offset, cache) == center - offset + radius
+        assert post.get_trigger(center + offset, cache) == center + offset - radius
+
+
+# Test pitch-tracking (spectrum)
+
+
 def test_correlate_offset():
     """
     Catches bug where writing N instead of Ncorr