diff --git a/CHANGES.rst b/CHANGES.rst
index 1db2ee834..ae5bc9f2f 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -23,7 +23,7 @@ datamodels
 - Fixed missing TUNITn keywords caused by changes for unsigned int columns. [#3753]
 
 - Write ``siaf_xref_sci`` and ``siaf_yref_sci`` to FITS keywords ``XREF_SCI``
-  and ``YREF_SCI`` for ``NRC_TSGRISM`` exposures. [#3766]
+  and ``YREF_SCI`` for ``NRC_TSGRISM`` exposures. [#3766
 
 extract_1d
 ----------
@@ -36,6 +36,11 @@ group_scale
 
 - Updates to documentation and log messages. [#3738]
 
+lib
+---
+
+- A function to determine the dispersion direction has been added. [#3756]
+
 stpipe
 ------
 
diff --git a/jwst/lib/dispaxis.py b/jwst/lib/dispaxis.py
new file mode 100644
index 000000000..74a255018
--- /dev/null
+++ b/jwst/lib/dispaxis.py
@@ -0,0 +1,152 @@
+import logging
+
+log = logging.getLogger(__name__)
+log.setLevel(logging.DEBUG)
+
+def get_dispersion_direction(exposure_type, grating="ANY", filter_wh="ANY",
+                             pupil="ANY"):
+    """Get the dispersion direction.
+
+    Parameters
+    ----------
+    exposure_type : str
+        The exposure type.
+
+    grating : str
+        The name of the optic in the grating wheel.
+
+    filter_wh : str
+        The name of the optic in the filter wheel.
+
+    pupil : str
+        The name of the optic in the pupil wheel.
+
+    Returns
+    -------
+    int or None : The dispersion direction
+
+-    None  the dispersion direction is not meaningful or not defined
+-       1  the dispersion direction is horizontal ("sky" coordinates)
+-       2  the dispersion direction is vertical ("sky" coordinates)
+    """
+
+    exposure_type = exposure_type.upper()
+    grating = grating.upper()
+    filter_wh = filter_wh.upper()
+    pupil = pupil.upper()
+
+    # The keys of the `by_exp_type` dictionary are values of exposure type.
+    # If the dispersion direction is uniquely determined by the exposure
+    # type, the associated value of by_exp_type[exposure_type] will be an
+    # integer, the dispersion direction.  If one or more parameters other
+    # than the exposure type are needed, the value of
+    # by_exp_type[exposure_type] will be a tuple of several strings; use
+    # that tuple as the key for dictionary `second_pass`, and the value of
+    # that will be the dispersion direction.
+    by_exp_type = {
+        # FGS science
+        "FGS_DARK": None,
+        "FGS_FOCUS": None,
+        "FGS_IMAGE": None,
+        "FGS_INTFLAT": None,
+        "FGS_SKYFLAT": None,
+        # FGS guide star
+        "FGS_ACQ1": None,
+        "FGS_ACQ2": None,
+        "FGS_FINEGUIDE": None,
+        "FGS_ID-IMAGE": None,
+        "FGS_ID-STACK": None,
+        "FGS_TRACK": None,
+        # MIRI
+        "MIR_4QPM": None,
+        "MIR_CORONCAL": None,
+        "MIR_DARKALL": None,
+        "MIR_DARKIMG": None,
+        "MIR_DARKMRS": None,
+        "MIR_FLATALL": None,
+        "MIR_FLATIMAGE": None,
+        "MIR_FLATIMAGE-EXT": None,
+        "MIR_FLATMRS": 2,
+        "MIR_FLATMRS-EXT": 2,
+        "MIR_IMAGE": None,
+        "MIR_LRS-FIXEDSLIT": 2,
+        "MIR_LRS-SLITLESS": 2,
+        "MIR_LYOT": None,
+        "MIR_MRS": 2,
+        "MIR_TACQ": None,
+        # NIRISS
+        "NIS_AMI": None,
+        "NIS_DARK": None,
+        "NIS_EXTCAL": None,
+        "NIS_FOCUS": None,
+        "NIS_IMAGE": None,
+        "NIS_LAMP": None,
+        "NIS_SOSS": 1,
+        "NIS_TACQ": None,
+        "NIS_TACONFIRM": None,
+        "NIS_WFSS": (exposure_type, "ANY", filter_wh, "ANY"),
+        # NIRCam
+        "NRC_CORON": None,
+        "NRC_DARK": None,
+        "NRC_FLAT": None,
+        "NRC_FOCUS": None,
+        "NRC_GRISM": (exposure_type, "ANY", "ANY", pupil),
+        "NRC_IMAGE": None,
+        "NRC_WFSS": (exposure_type, "ANY", "ANY", pupil),
+        "NRC_LED": None,
+        "NRC_WFSC": None,
+        "NRC_TACONFIRM": None,
+        "NRC_TACQ": None,
+        "NRC_TSGRISM": (exposure_type, "ANY", "ANY", pupil),
+        "NRC_TSIMAGE": None,
+        # NIRSpec
+        "NRS_AUTOFLAT": None,
+        "NRS_AUTOWAVE": 1,
+        "NRS_BRIGHTOBJ": 1,
+        "NRS_CONFIRM": None,
+        "NRS_DARK": None,
+        "NRS_FIXEDSLIT": 1,
+        "NRS_FOCUS": None,
+        "NRS_IFU": 1,
+        "NRS_IMAGE": None,
+        "NRS_LAMP":  1,
+        "NRS_MIMF": None,
+        "NRS_MSASPEC": 1,
+        "NRS_MSATA": None,
+        "NRS_TACONFIRM": None,
+        "NRS_TACQ": None,
+        "NRS_TASLIT": None,
+        "NRS_WATA": None,
+        # Misc
+        "N/A": None,
+        "ANY": None
+    }
+
+    if exposure_type not in by_exp_type.keys():
+        return None
+
+    # The strings in each tuple are exposure_type, grating, filter_wh, pupil.
+    second_pass = {
+        ("NIS_WFSS", "ANY", "GR150R", "ANY"): 2,
+        ("NIS_WFSS", "ANY", "GR150C", "ANY"): 1,
+
+        ("NRC_GRISM", "ANY", "ANY", "GRISMR"): 1,
+        ("NRC_GRISM", "ANY", "ANY", "GRISMC"): 2,
+
+        ("NRC_TSGRISM", "ANY", "ANY", "GRISMR"): 1,
+
+        ("NRC_WFSS", "ANY", "ANY", "GRISMR"): 1,
+        ("NRC_WFSS", "ANY", "ANY", "GRISMC"): 2
+    }
+
+    select = by_exp_type[exposure_type]
+    if isinstance(select, int):
+        return select
+    else:
+        if select in second_pass.keys():
+            return second_pass[select]
+        else:
+            log.warning("Error in get_dispersion_direction:  {} not in "
+                        "`second_pass`".format(select))
+            log.warning("Dispersion direction could not be determined.")
+            return None

diff --git a/jwst/lib/tests/test_dispaxis.py b/jwst/lib/tests/test_dispaxis.py
new file mode 100644
index 000000000..17b328f27
--- /dev/null
+++ b/jwst/lib/tests/test_dispaxis.py
@@ -0,0 +1,85 @@
+"""
+Test for dispaxis
+"""
+
+from .. import dispaxis
+
+def test_dispaxis_1():
+    value = dispaxis.get_dispersion_direction("FGS_IMAGE", "junk", "junk",
+                                              "junk")
+    assert value == None
+
+def test_dispaxis_2():
+    value = dispaxis.get_dispersion_direction("MIR_4QPM", "junk", "junk",
+                                              "junk")
+    assert value == None
+
+def test_dispaxis_3():
+    value = dispaxis.get_dispersion_direction("MIR_LRS-SLITLESS", "junk",
+                                              "junk", "junk")
+    assert value == 2
+
+def test_dispaxis_4():
+    value = dispaxis.get_dispersion_direction("MIR_LRS-FIXEDSLIT", "junk",
+                                              "junk", "junk")
+    assert value == 2
+
+def test_dispaxis_5():
+    value = dispaxis.get_dispersion_direction("NIS_SOSS", "junk", "junk",
+                                              "junk")
+    assert value == 1
+
+def test_dispaxis_6():
+    value = dispaxis.get_dispersion_direction("NRS_FIXEDSLIT", "junk", "junk",
+                                              "junk")
+    assert value == 1
+
+def test_dispaxis_7():
+    value = dispaxis.get_dispersion_direction("NRS_IFU", "junk", "junk",
+                                              "junk")
+    assert value == 1
+
+def test_dispaxis_8():
+    value = dispaxis.get_dispersion_direction("NRS_MSASPEC", "junk", "junk",
+                                              "junk")
+    assert value == 1
+
+def test_dispaxis_9():
+    value = dispaxis.get_dispersion_direction("NIS_WFSS", "junk", "GR150R",
+                                              "junk")
+    assert value == 2
+
+def test_dispaxis_10():
+    value = dispaxis.get_dispersion_direction("NIS_WFSS", "junk", "GR150C",
+                                              "junk")
+    assert value == 1
+
+def test_dispaxis_11():
+    value = dispaxis.get_dispersion_direction("NRC_GRISM", "junk", "junk",
+                                              "GRISMR")
+    assert value == 1
+
+def test_dispaxis_12():
+    value = dispaxis.get_dispersion_direction("NRC_GRISM", "junk", "junk",
+                                              "GRISMC")
+    assert value == 2
+
+def test_dispaxis_13():
+    value = dispaxis.get_dispersion_direction("NRC_TSGRISM", "junk", "junk",
+                                              "GRISMR")
+    assert value == 1
+
+def test_dispaxis_14():
+    value = dispaxis.get_dispersion_direction("NRC_WFSS", "junk", "junk",
+                                              "GRISMR")
+    assert value == 1
+
+def test_dispaxis_15():
+    value = dispaxis.get_dispersion_direction("NRC_WFSS", "junk", "junk",
+                                              "GRISMC")
+    assert value == 2
+
+def test_dispaxis_16():
+    value = dispaxis.get_dispersion_direction("nrc_grism", "junk", "junk",
+                                              "missing")
+    assert value == None

Write a function to obtain the dispersion direction
Issue [JP-774](https://jira.stsci.edu/browse/JP-774) was created by Philip Hodge:

For spectroscopic data, there should be a function to determine the dispersion direction, based on instrument mode.
jwst/lib/tests/test_dispaxis.py::test_dispaxis_1 jwst/lib/tests/test_dispaxis.py::test_dispaxis_2 jwst/lib/tests/test_dispaxis.py::test_dispaxis_3 jwst/lib/tests/test_dispaxis.py::test_dispaxis_4 jwst/lib/tests/test_dispaxis.py::test_dispaxis_5 jwst/lib/tests/test_dispaxis.py::test_dispaxis_6 jwst/lib/tests/test_dispaxis.py::test_dispaxis_7 jwst/lib/tests/test_dispaxis.py::test_dispaxis_8 jwst/lib/tests/test_dispaxis.py::test_dispaxis_9 jwst/lib/tests/test_dispaxis.py::test_dispaxis_10 jwst/lib/tests/test_dispaxis.py::test_dispaxis_11 jwst/lib/tests/test_dispaxis.py::test_dispaxis_12 jwst/lib/tests/test_dispaxis.py::test_dispaxis_13 jwst/lib/tests/test_dispaxis.py::test_dispaxis_14 jwst/lib/tests/test_dispaxis.py::test_dispaxis_15 jwst/lib/tests/test_dispaxis.py::test_dispaxis_16