diff --git a/eva_vcf_merge/merge.py b/eva_vcf_merge/merge.py
index cc04954..32adcbb 100644
--- a/eva_vcf_merge/merge.py
+++ b/eva_vcf_merge/merge.py
@@ -17,8 +17,9 @@
 
 from ebi_eva_common_pyutils.nextflow import NextFlowPipeline, NextFlowProcess
 
-from eva_vcf_merge.multistage import get_multistage_vertical_concat_pipeline
-from eva_vcf_merge.utils import write_files_to_list, get_valid_filename, validate_aliases
+from eva_vcf_merge.detect import MergeType
+from eva_vcf_merge.multistage import get_multistage_merge_pipeline
+from eva_vcf_merge.utils import get_valid_filename, validate_aliases
 
 
 class VCFMerger:
@@ -30,32 +31,17 @@ def __init__(self, bgzip_binary, bcftools_binary, nextflow_binary, nextflow_conf
         self.output_dir = output_dir
         self.working_dir = os.path.join(output_dir, 'nextflow')
 
-    def horizontal_merge(self, vcf_groups, resume=True):
-        """
-        Merge groups of vcfs horizontally, i.e. by sample, using bcftools.
-
-        :param vcf_groups: dict mapping a string (e.g. an analysis alias) to a group of vcf files to be merged
-        :param resume: whether to resume pipeline (default true)
-        :returns: dict of merged filenames
-        """
-        if not validate_aliases(vcf_groups.keys()):
-            raise ValueError('Aliases must be unique when converted to filenames')
-        pipeline, merged_filenames = self.generate_horizontal_merge_pipeline(vcf_groups)
-        workflow_file = os.path.join(self.working_dir, 'horizontal_merge.nf')
-        os.makedirs(self.working_dir, exist_ok=True)
-        pipeline.run_pipeline(
-            workflow_file_path=workflow_file,
-            working_dir=self.working_dir,
-            nextflow_binary_path=self.nextflow_binary,
-            nextflow_config_path=self.nextflow_config,
-            resume=resume
-        )
-        return merged_filenames
+    def horizontal_merge(self, vcf_groups, chunk_size=500, resume=True):
+        return self.common_merge(MergeType.HORIZONTAL, vcf_groups, chunk_size, resume)
 
     def vertical_merge(self, vcf_groups, chunk_size=500, resume=True):
+        return self.common_merge(MergeType.VERTICAL, vcf_groups, chunk_size, resume)
+
+    def common_merge(self, merge_type, vcf_groups, chunk_size=500, resume=True):
         """
-        Merge groups of vcfs vertically, i.e. concatenation.
+        Merge groups of vcfs horizontally or vertically.
 
+        :param merge_type: vertical or horizontal merge
         :param vcf_groups: dict mapping a string (e.g. an analysis alias) to a group of vcf files to be merged
         :param chunk_size: number of vcfs to merge at once (default 500)
         :param resume: whether to resume pipeline (default true)
@@ -63,8 +49,9 @@ def vertical_merge(self, vcf_groups, chunk_size=500, resume=True):
         """
         if not validate_aliases(vcf_groups.keys()):
             raise ValueError('Aliases must be unique when converted to filenames')
-        pipeline, merged_filenames = self.generate_vertical_merge_pipeline(vcf_groups, chunk_size)
-        workflow_file = os.path.join(self.working_dir, "vertical_concat.nf")
+        pipeline, merged_filenames = self.generate_merge_pipeline(merge_type, vcf_groups, chunk_size)
+        workflow_file = os.path.join(self.working_dir, "merge.nf")
+
         os.makedirs(self.working_dir, exist_ok=True)
         pipeline.run_pipeline(
             workflow_file_path=workflow_file,
@@ -81,37 +68,11 @@ def vertical_merge(self, vcf_groups, chunk_size=500, resume=True):
             merged_filenames[alias] = target_filename
         return merged_filenames
 
-    def generate_horizontal_merge_pipeline(self, vcf_groups):
+    def generate_merge_pipeline(self, merge_type, vcf_groups, chunk_size):
         """
-        Generate horizontal merge pipeline, including compressing and indexing VCFs.
-
-        :param vcf_groups: dict mapping a string to a group of vcf files to be merged
-        :return: complete NextflowPipeline and dict of merged filenames
-        """
-        dependencies = {}
-        merged_filenames = {}
-        for alias_idx, (alias, vcfs) in enumerate(vcf_groups.items()):
-            deps, index_processes, compressed_vcfs = self.compress_and_index(alias_idx, vcfs)
-            dependencies.update(deps)
-
-            safe_alias = get_valid_filename(alias)
-            list_filename = write_files_to_list(compressed_vcfs, safe_alias, self.working_dir)
-            merged_filename = os.path.join(self.output_dir, f'{safe_alias}_merged.vcf.gz')
-            merge_process = NextFlowProcess(
-                process_name=f'merge_{alias_idx}',
-                command_to_run=f'{self.bcftools_binary} merge --merge all --file-list {list_filename} '
-                               f'--threads 3 -O z -o {merged_filename}'
-            )
-            # each alias's merge process depends on all index processes
-            dependencies[merge_process] = index_processes
-            merged_filenames[alias] = merged_filename
-
-        return NextFlowPipeline(dependencies), merged_filenames
-
-    def generate_vertical_merge_pipeline(self, vcf_groups, chunk_size):
-        """
-        Generate vertical merge (concatenation) pipeline.
+        Generate merge pipeline, including compressing and indexing VCFs.
 
+        :param merge_type: vertical or horizontal merge
         :param vcf_groups: dict mapping a string to a group of vcf files to be merged
         :param chunk_size: number of vcfs to merge at once
         :return: complete NextFlowPipeline and dict of merged filenames
@@ -119,16 +80,28 @@ def generate_vertical_merge_pipeline(self, vcf_groups, chunk_size):
         full_pipeline = NextFlowPipeline()
         merged_filenames = {}
         for alias_idx, (alias, vcfs) in enumerate(vcf_groups.items()):
-            deps, index_processes, compressed_vcfs = self.compress_and_index(alias_idx, vcfs)
-            compress_pipeline = NextFlowPipeline(deps)
-            concat_pipeline, merged_filename = get_multistage_vertical_concat_pipeline(
-                alias=alias_idx,
-                vcf_files=compressed_vcfs,
-                concat_chunk_size=chunk_size,
-                concat_processing_dir=self.working_dir,
-                bcftools_binary=self.bcftools_binary
-            )
-            pipeline = NextFlowPipeline.join_pipelines(compress_pipeline, concat_pipeline)
+            compress_pipeline, compressed_vcfs = self.compress_and_index(alias_idx, vcfs)
+            if merge_type == MergeType.HORIZONTAL:
+                merge_pipeline, merged_filename = get_multistage_merge_pipeline(
+                    alias=alias_idx,
+                    vcf_files=compressed_vcfs,
+                    chunk_size=chunk_size,
+                    processing_dir=self.working_dir,
+                    bcftools_binary=self.bcftools_binary,
+                    process_name='merge',
+                    process_command=self.merge_command
+                )
+            else:
+                merge_pipeline, merged_filename = get_multistage_merge_pipeline(
+                    alias=alias_idx,
+                    vcf_files=compressed_vcfs,
+                    chunk_size=chunk_size,
+                    processing_dir=self.working_dir,
+                    bcftools_binary=self.bcftools_binary,
+                    process_name='concat',
+                    process_command=self.concat_command
+                )
+            pipeline = NextFlowPipeline.join_pipelines(compress_pipeline, merge_pipeline)
             full_pipeline = NextFlowPipeline.join_pipelines(full_pipeline, pipeline)
             merged_filenames[alias] = merged_filename
         return full_pipeline, merged_filenames
@@ -139,7 +112,7 @@ def compress_and_index(self, alias, vcfs):
 
         :param alias: name of group of vcf files (used to name Nextflow processes uniquely)
         :param vcfs: list of vcf files
-        :return: dependency map, list of final index processes, and list of final filenames
+        :return: NextFlow pipeline and list of final filenames
         """
         dependencies = {}
         index_processes = []
@@ -160,5 +133,12 @@ def compress_and_index(self, alias, vcfs):
             index_processes.append(index_process)
             # each file's index depends only on compress (if present)
             dependencies[index_process] = [compress_process] if compress_process else []
-        # TODO preferably return a NextFlowPipeline rather than dependencies & final processes
-        return dependencies, index_processes, compressed_vcfs
+        return NextFlowPipeline(dependencies), compressed_vcfs
+
+    def merge_command(self, files_to_merge_list, output_vcf_file):
+        return (f'{self.bcftools_binary} merge --merge all --file-list {files_to_merge_list} --threads 3 '
+                f'-O z -o {output_vcf_file}')
+
+    def concat_command(self, files_to_merge_list, output_vcf_file):
+        return (f'{self.bcftools_binary} concat --allow-overlaps --remove-duplicates --file-list {files_to_merge_list} '
+                f'-O z -o {output_vcf_file}')
diff --git a/eva_vcf_merge/multistage.py b/eva_vcf_merge/multistage.py
index 72ee8d2..5f1f3e4 100644
--- a/eva_vcf_merge/multistage.py
+++ b/eva_vcf_merge/multistage.py
@@ -17,22 +17,24 @@
 
 from ebi_eva_common_pyutils.nextflow import NextFlowPipeline, NextFlowProcess
 
-from eva_vcf_merge.utils import write_files_to_list
 
-
-def get_multistage_vertical_concat_pipeline(
+def get_multistage_merge_pipeline(
         alias,
         vcf_files,
-        concat_processing_dir,
-        concat_chunk_size,
+        processing_dir,
+        chunk_size,
         bcftools_binary,
+        process_name,
+        process_command,
         stage=0,
         prev_stage_processes=None,
         pipeline=None
 ):
     """
-    # Generate Nextflow pipeline for multi-stage VCF concatenation of 5 VCF files with 2-VCFs concatenated at a time (CONCAT_CHUNK_SIZE=2)
-    # For illustration purposes only. Usually the CONCAT_CHUNK_SIZE is much higher (ex: 500).
+    Generate Nextflow pipeline for multi-stage VCF merging or concatenation.
+
+    # As an example, below is multi-stage VCF concatenation of 5 VCF files with 2-VCFs concatenated at a time (CHUNK_SIZE=2)
+    # For illustration purposes only. Usually the CHUNK_SIZE is much higher (ex: 500).
     #
     #		    vcf1		    vcf2		vcf3		    vcf4		vcf5
     #               \		     /		       \		      /
@@ -51,70 +53,75 @@ def get_multistage_vertical_concat_pipeline(
     if not pipeline:
         pipeline = NextFlowPipeline()
         prev_stage_processes = []
-    # If we are left with only one file, this means we have reached the last concat stage
+    # If we are left with only one file, this means we have reached the last merge stage
     if len(vcf_files) == 1:
         return pipeline, vcf_files[0]
 
-    num_batches_in_stage = math.ceil(len(vcf_files) / concat_chunk_size)
+    num_batches_in_stage = math.ceil(len(vcf_files) / chunk_size)
     curr_stage_processes = []
     output_vcf_files_from_stage = []
     for batch in range(0, num_batches_in_stage):
-        # split files in the current stage into chunks based on concat_chunk_size
-        files_in_batch = vcf_files[(concat_chunk_size * batch):(concat_chunk_size * (batch + 1))]
-        files_to_concat_list = write_files_to_concat_list(files_in_batch, stage, batch, concat_processing_dir)
-        output_vcf_file = get_output_vcf_file_name(alias, stage, batch, concat_processing_dir)
+        # split files in the current stage into chunks based on chunk_size
+        files_in_batch = vcf_files[(chunk_size * batch):(chunk_size * (batch + 1))]
+        files_to_merge_list = write_files_to_merge_list(process_name, alias, files_in_batch, stage, batch, processing_dir)
+        output_vcf_file = get_output_vcf_file_name(process_name, alias, stage, batch, processing_dir)
 
-        # separate concat & index processes
-        concat_process = NextFlowProcess(
-            process_name=f"concat{alias}_stage{stage}_batch{batch}",
-            command_to_run=f"{bcftools_binary} concat --allow-overlaps --remove-duplicates "
-                           f"--file-list {files_to_concat_list} -o {output_vcf_file} -O z"
+        # separate merge & index processes
+        merge_process = NextFlowProcess(
+            process_name=f"{process_name}{alias}_stage{stage}_batch{batch}",
+            command_to_run=process_command(files_to_merge_list, output_vcf_file)
         )
         index_process = NextFlowProcess(
             process_name=f"index{alias}_stage{stage}_batch{batch}",
             command_to_run=f"{bcftools_binary} index --csi {output_vcf_file}"
         )
-        # index depends only on this batch's concat
-        pipeline.add_dependencies({index_process: [concat_process]})
+        # index depends only on this batch's merge
+        pipeline.add_dependencies({index_process: [merge_process]})
         # next stage requires indexing to be complete from this stage
         curr_stage_processes.append(index_process)
         output_vcf_files_from_stage.append(output_vcf_file)
 
-        # Concatenation batch in a given stage will have to wait until the completion of
-        # n batches in the previous stage where n = concat_chunk_size
+        # Merge batch in a given stage will have to wait until the completion of
+        # n batches in the previous stage where n = chunk_size
         # Ex: In the illustration above stage 1/batch 0 depends on completion of stage 0/batch 0 and stage 0/batch 1
         # While output of any n batches from the previous stage can be worked on as they become available,
         # having a predictable formula simplifies pipeline generation and troubleshooting
-        prev_stage_dependencies = prev_stage_processes[(concat_chunk_size * batch):(concat_chunk_size * (batch + 1))]
-        pipeline.add_dependencies({concat_process: prev_stage_dependencies})
+        prev_stage_dependencies = prev_stage_processes[(chunk_size * batch):(chunk_size * (batch + 1))]
+        pipeline.add_dependencies({merge_process: prev_stage_dependencies})
     prev_stage_processes = curr_stage_processes
 
-    return get_multistage_vertical_concat_pipeline(
+    return get_multistage_merge_pipeline(
         alias, output_vcf_files_from_stage,
-        concat_processing_dir, concat_chunk_size,
+        processing_dir, chunk_size,
         bcftools_binary,
+        process_name,
+        process_command,
         stage=stage+1,
         prev_stage_processes=prev_stage_processes,
         pipeline=pipeline
     )
 
 
-def write_files_to_concat_list(files_to_concat, concat_stage, concat_batch, concat_processing_dir):
+def write_files_to_merge_list(process_name, alias, files_to_merge, stage, batch, processing_dir):
     """
-    Write the list of files to be concatenated for a given stage and batch
+    Write the list of files to be merged for a given stage and batch
     """
-    output_dir = get_concat_output_dir(concat_stage, concat_processing_dir)
-    alias = f'batch{concat_batch}'
-    return write_files_to_list(files_to_concat, alias, output_dir)
+    output_dir = get_output_dir(process_name, alias, stage, processing_dir)
+    list_filename = os.path.join(output_dir, f"batch{batch}_files.list")
+    os.makedirs(os.path.dirname(list_filename), exist_ok=True)
+    with open(list_filename, "w") as handle:
+        for filename in files_to_merge:
+            handle.write(filename + "\n")
+    return list_filename
 
 
-def get_concat_output_dir(concat_stage_index: int, concat_processing_dir: str):
+def get_output_dir(process_name, alias, stage_index, processing_dir):
     """
-    Get the file name with the list of files to be concatenated for a given stage and batch in the concatenation process
+    Get the file name with the list of files to be merged for a given stage and batch in the merge process
     """
-    return os.path.join(concat_processing_dir, "vertical_concat", f"stage_{concat_stage_index}")
+    return os.path.join(processing_dir, f'{process_name}_{alias}', f"stage_{stage_index}")
 
 
-def get_output_vcf_file_name(alias: str, concat_stage_index: int, concat_batch_index: int, concat_processing_dir: str):
-    return os.path.join(get_concat_output_dir(concat_stage_index, concat_processing_dir),
-                        f"concat{alias}_output_stage{concat_stage_index}_batch{concat_batch_index}.vcf.gz")
+def get_output_vcf_file_name(process_name, alias, stage_index, batch_index, processing_dir):
+    return os.path.join(get_output_dir(process_name, alias, stage_index, processing_dir),
+                        f"{process_name}{alias}_output_stage{stage_index}_batch{batch_index}.vcf.gz")
diff --git a/eva_vcf_merge/utils.py b/eva_vcf_merge/utils.py
index 850ec22..53d0b67 100644
--- a/eva_vcf_merge/utils.py
+++ b/eva_vcf_merge/utils.py
@@ -26,16 +26,6 @@ def are_all_elements_unique(elements):
     return True
 
 
-def write_files_to_list(files, alias, output_dir):
-    """Write the list of files to a path built from alias and output_dir."""
-    list_filename = os.path.join(output_dir, f"{alias}_files.list")
-    os.makedirs(os.path.dirname(list_filename), exist_ok=True)
-    with open(list_filename, "w") as handle:
-        for filename in files:
-            handle.write(filename + "\n")
-    return list_filename
-
-
 def get_valid_filename(s):
     """Return string with characters not allowed in filenames replaced by underscore."""
     return re.sub(r'[^-.0-9a-zA-Z]+', '_', s)
diff --git a/tests/test_merge.py b/tests/test_merge.py
index 3a1988a..5f50f4f 100644
--- a/tests/test_merge.py
+++ b/tests/test_merge.py
@@ -63,7 +63,7 @@ def test_concat_uninterrupted(vcf_merger, many_vcfs_to_concat):
     #                                 Final_merged                      <-------- Stage 2
     vcfs = {'many': many_vcfs_to_concat}
     vcf_merger.vertical_merge(vcfs, chunk_size=2, resume=False)
-    stage_dirs = glob.glob(f'{vcf_merger.working_dir}/vertical_concat/stage*')
+    stage_dirs = glob.glob(f'{vcf_merger.working_dir}/concat_0/stage*')
     assert len(stage_dirs) == 3
     output_vcf_from_multi_stage_concat = os.path.join(vcf_merger.output_dir, 'many_merged.vcf.gz')
     output_vcf_from_single_stage_concat = f'{vcf_merger.output_dir}/single_stage_concat_result.vcf.gz'