diff --git a/sae/config.py b/sae/config.py
index 12d91d2..31c6e1d 100644
--- a/sae/config.py
+++ b/sae/config.py
@@ -67,14 +67,15 @@ class TrainConfig(Serializable):
     save_every: int = 1000
     """Save SAEs every `save_every` steps."""
 
-    instance_dims: tuple = (0, 1)
+    sample_dims: set[int] = {0, 1}
+    """Dimensions containing SAE inputs."""
 
-    feature_dims: tuple = (2,)
+    feature_dims: set[int] = {2}
+    """Dimensions of SAE inputs."""
     
     log_to_wandb: bool = True
     run_name: str | None = None
     wandb_log_frequency: int = 1
-    
 
     def __post_init__(self):
         assert not (
diff --git a/sae/trainer.py b/sae/trainer.py
index f277f65..0026e04 100644
--- a/sae/trainer.py
+++ b/sae/trainer.py
@@ -248,11 +248,11 @@ def hook(module: nn.Module, inputs, outputs):
                 inputs = input_dict.get(name, outputs)
                 raw = self.saes[name]           # 'raw' never has a DDP wrapper
 
-                outputs = outputs.permute(*self.cfg.instance_dims, *self.cfg.feature_dims)
+                outputs = outputs.permute(*self.cfg.sample_dims, *self.cfg.feature_dims)
                 outputs = outputs.reshape(-1, raw.d_in)
 
                 if self.cfg.transcode:
-                    inputs = inputs.permute(*self.cfg.instance_dims, *self.cfg.feature_dims)
+                    inputs = inputs.permute(*self.cfg.sample_dims, *self.cfg.feature_dims)
                     inputs = inputs.reshape(-1, raw.d_in)
 
                 # On the first iteration, initialize the decoder bias
diff --git a/sae/utils.py b/sae/utils.py
index cac2f0a..4d5e483 100644
--- a/sae/utils.py
+++ b/sae/utils.py
@@ -63,7 +63,7 @@ def get_layer_list(model: PreTrainedModel) -> tuple[str, nn.ModuleList]:
 @torch.inference_mode()
 def resolve_widths(
     model: PreTrainedModel, module_names: list[str], dummy_inputs: dict[str, Tensor], 
-    dims: tuple[int] = (-1,),
+    dims: set[int] = {-1},
 ) -> dict[str, int]:
     """Find number of output dimensions for the specified modules."""
     module_to_name = {