diff --git a/Project.toml b/Project.toml
index d0aab94f..613811f7 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "BlockSparseArrays"
 uuid = "2c9a651f-6452-4ace-a6ac-809f4280fbb4"
 authors = ["ITensor developers <support@itensor.org> and contributors"]
-version = "0.7.5"
+version = "0.7.6"
 
 [deps]
 Adapt = "79e6a3ab-5dfb-504d-930d-738a2a938a0e"
@@ -31,7 +31,7 @@ Adapt = "4.1.1"
 Aqua = "0.8.9"
 ArrayLayouts = "1.10.4"
 BlockArrays = "1.2.0"
-DerivableInterfaces = "0.5"
+DerivableInterfaces = "0.5.2"
 DiagonalArrays = "0.3"
 Dictionaries = "0.4.3"
 FillArrays = "1.13.0"
@@ -44,7 +44,7 @@ SparseArraysBase = "0.5"
 SplitApplyCombine = "1.2.3"
 TensorAlgebra = "0.3.2"
 Test = "1.10"
-TypeParameterAccessors = "0.2.0, 0.3"
+TypeParameterAccessors = "0.4"
 julia = "1.10"
 
 [extras]
diff --git a/src/BlockArraysExtensions/blockedunitrange.jl b/src/BlockArraysExtensions/blockedunitrange.jl
index 2e916729..35779057 100644
--- a/src/BlockArraysExtensions/blockedunitrange.jl
+++ b/src/BlockArraysExtensions/blockedunitrange.jl
@@ -29,10 +29,16 @@ axis(a::AbstractVector) = axes(a, 1)
 function eachblockaxis(a::AbstractVector)
   return map(axis, blocks(a))
 end
+function blockaxistype(a::AbstractVector)
+  return eltype(eachblockaxis(a))
+end
 
 # Take a collection of axes and mortar them
 # into a single blocked axis.
 function mortar_axis(axs)
+  return blockrange(axs)
+end
+function mortar_axis(axs::Vector{<:Base.OneTo{<:Integer}})
   return blockedrange(length.(axs))
 end
 
diff --git a/src/abstractblocksparsearray/abstractblocksparsearray.jl b/src/abstractblocksparsearray/abstractblocksparsearray.jl
index e90e97e0..f82e2ca9 100644
--- a/src/abstractblocksparsearray/abstractblocksparsearray.jl
+++ b/src/abstractblocksparsearray/abstractblocksparsearray.jl
@@ -19,12 +19,12 @@ end
 
 # Specialized in order to fix ambiguity error with `BlockArrays`.
 function Base.getindex(a::AbstractBlockSparseArray{<:Any,N}, I::Vararg{Int,N}) where {N}
-  return @interface BlockSparseArrayInterface() getindex(a, I...)
+  return @interface interface(a) getindex(a, I...)
 end
 
 # Specialized in order to fix ambiguity error with `BlockArrays`.
 function Base.getindex(a::AbstractBlockSparseArray{<:Any,0})
-  return @interface BlockSparseArrayInterface() getindex(a)
+  return @interface interface(a) getindex(a)
 end
 
 ## # Fix ambiguity error with `BlockArrays`.
@@ -39,7 +39,7 @@ end
 ##
 ## # Fix ambiguity error with `BlockArrays`.
 ## function Base.getindex(a::AbstractBlockSparseArray, I::Vararg{AbstractVector})
-##   ## return @interface BlockSparseArrayInterface() getindex(a, I...)
+##   ## return @interface interface(a) getindex(a, I...)
 ##   return ArrayLayouts.layout_getindex(a, I...)
 ## end
 
@@ -47,13 +47,13 @@ end
 function Base.setindex!(
   a::AbstractBlockSparseArray{<:Any,N}, value, I::Vararg{Int,N}
 ) where {N}
-  @interface BlockSparseArrayInterface() setindex!(a, value, I...)
+  @interface interface(a) setindex!(a, value, I...)
   return a
 end
 
 # Fix ambiguity error.
 function Base.setindex!(a::AbstractBlockSparseArray{<:Any,0}, value)
-  @interface BlockSparseArrayInterface() setindex!(a, value)
+  @interface interface(a) setindex!(a, value)
   return a
 end
 
diff --git a/src/abstractblocksparsearray/arraylayouts.jl b/src/abstractblocksparsearray/arraylayouts.jl
index b875c5d9..684fe646 100644
--- a/src/abstractblocksparsearray/arraylayouts.jl
+++ b/src/abstractblocksparsearray/arraylayouts.jl
@@ -27,9 +27,8 @@ function Base.similar(
   elt::Type,
   axes,
 ) where {A,B}
-  # TODO: Check that this equals `similartype(blocktype(B), elt, axes)`,
-  # or maybe promote them?
-  output_blocktype = similartype(blocktype(A), elt, axes)
+  # TODO: Use something like `Base.promote_op(*, A, B)` to determine the output block type.
+  output_blocktype = similartype(blocktype(A), Type{elt}, Tuple{blockaxistype.(axes)...})
   return similar(BlockSparseArray{elt,length(axes),output_blocktype}, axes)
 end
 
diff --git a/src/abstractblocksparsearray/unblockedsubarray.jl b/src/abstractblocksparsearray/unblockedsubarray.jl
index fc80e92f..a10d5419 100644
--- a/src/abstractblocksparsearray/unblockedsubarray.jl
+++ b/src/abstractblocksparsearray/unblockedsubarray.jl
@@ -30,10 +30,6 @@ function Broadcast.BroadcastStyle(arraytype::Type{<:UnblockedSubArray})
   return BroadcastStyle(blocktype(parenttype(arraytype)))
 end
 
-function TypeParameterAccessors.similartype(arraytype::Type{<:UnblockedSubArray}, elt::Type)
-  return similartype(blocktype(parenttype(arraytype)), elt)
-end
-
 function Base.similar(
   a::UnblockedSubArray, elt::Type, axes::Tuple{Base.OneTo,Vararg{Base.OneTo}}
 )
diff --git a/src/abstractblocksparsearray/wrappedabstractblocksparsearray.jl b/src/abstractblocksparsearray/wrappedabstractblocksparsearray.jl
index b72f1e41..86425925 100644
--- a/src/abstractblocksparsearray/wrappedabstractblocksparsearray.jl
+++ b/src/abstractblocksparsearray/wrappedabstractblocksparsearray.jl
@@ -29,8 +29,8 @@ const AnyAbstractBlockSparseVecOrMat{T,N} = Union{
   AnyAbstractBlockSparseVector{T},AnyAbstractBlockSparseMatrix{T}
 }
 
-function DerivableInterfaces.interface(::Type{<:AnyAbstractBlockSparseArray})
-  return BlockSparseArrayInterface()
+function DerivableInterfaces.interface(arrayt::Type{<:AnyAbstractBlockSparseArray})
+  return BlockSparseArrayInterface(interface(blocktype(arrayt)))
 end
 
 # a[1:2, 1:2]
@@ -231,9 +231,9 @@ function Base.similar(
 end
 
 function blocksparse_similar(a, elt::Type, axes::Tuple)
-  return BlockSparseArray{elt,length(axes),similartype(blocktype(a), elt, axes)}(
-    undef, axes
-  )
+  ndims = length(axes)
+  blockt = similartype(blocktype(a), Type{elt}, Tuple{blockaxistype.(axes)...})
+  return BlockSparseArray{elt,ndims,blockt}(undef, axes)
 end
 @interface ::AbstractBlockSparseArrayInterface function Base.similar(
   a::AbstractArray, elt::Type, axes::Tuple{Vararg{Int}}
diff --git a/src/blocksparsearrayinterface/blocksparsearrayinterface.jl b/src/blocksparsearrayinterface/blocksparsearrayinterface.jl
index 5c1ed5be..b4ce4038 100644
--- a/src/blocksparsearrayinterface/blocksparsearrayinterface.jl
+++ b/src/blocksparsearrayinterface/blocksparsearrayinterface.jl
@@ -16,7 +16,13 @@ using BlockArrays:
   blocklength,
   blocks,
   findblockindex
-using DerivableInterfaces: DerivableInterfaces, @interface, DefaultArrayInterface, zero!
+using DerivableInterfaces:
+  DerivableInterfaces,
+  @interface,
+  AbstractArrayInterface,
+  DefaultArrayInterface,
+  interface,
+  zero!
 using LinearAlgebra: Adjoint, Transpose
 using SparseArraysBase:
   AbstractSparseArrayInterface,
@@ -101,18 +107,47 @@ blockstype(a::BlockArray) = blockstype(typeof(a))
 blocktype(arraytype::Type{<:BlockArray}) = eltype(blockstype(arraytype))
 blocktype(a::BlockArray) = eltype(blocks(a))
 
-abstract type AbstractBlockSparseArrayInterface{N} <: AbstractSparseArrayInterface{N} end
+abstract type AbstractBlockSparseArrayInterface{N,B<:AbstractArrayInterface{N}} <:
+              AbstractSparseArrayInterface{N} end
+
+function blockinterface(interface::AbstractBlockSparseArrayInterface{<:Any,B}) where {B}
+  return B()
+end
 
 # TODO: Also support specifying the `blocktype` along with the `eltype`.
-function Base.similar(::AbstractBlockSparseArrayInterface, T::Type, ax::Tuple)
-  return similar(BlockSparseArray{T}, ax)
+function Base.similar(interface::AbstractBlockSparseArrayInterface, T::Type, ax::Tuple)
+  # TODO: Generalize by storing the block interface in the block sparse array interface.
+  N = length(ax)
+  B = similartype(typeof(blockinterface(interface)), Type{T}, Tuple{blockaxistype.(ax)...})
+  return similar(BlockSparseArray{T,N,B}, ax)
 end
 
-struct BlockSparseArrayInterface{N} <: AbstractBlockSparseArrayInterface{N} end
+struct BlockSparseArrayInterface{N,B<:AbstractArrayInterface{N}} <:
+       AbstractBlockSparseArrayInterface{N,B}
+  blockinterface::B
+end
+function BlockSparseArrayInterface{N}(blockinterface::AbstractArrayInterface{N}) where {N}
+  return BlockSparseArrayInterface{N,typeof(blockinterface)}(blockinterface)
+end
+function BlockSparseArrayInterface{M,B}(::Val{N}) where {M,B<:AbstractArrayInterface{M},N}
+  B′ = B(Val(N))
+  return BlockSparseArrayInterface(B′)
+end
+function BlockSparseArrayInterface{N}() where {N}
+  return BlockSparseArrayInterface{N}(DefaultArrayInterface{N}())
+end
 BlockSparseArrayInterface(::Val{N}) where {N} = BlockSparseArrayInterface{N}()
 BlockSparseArrayInterface{M}(::Val{N}) where {M,N} = BlockSparseArrayInterface{N}()
 BlockSparseArrayInterface() = BlockSparseArrayInterface{Any}()
 
+function DerivableInterfaces.combine_interface_rule(
+  interface1::AbstractBlockSparseArrayInterface,
+  interface2::AbstractBlockSparseArrayInterface,
+)
+  B = interface(blockinterface(interface1), blockinterface(interface2))
+  return BlockSparseArrayInterface(B)
+end
+
 @interface ::AbstractBlockSparseArrayInterface function BlockArrays.blocks(a::AbstractArray)
   return error("Not implemented")
 end
diff --git a/test/Project.toml b/test/Project.toml
index 9ce1d39a..8c52cd72 100644
--- a/test/Project.toml
+++ b/test/Project.toml
@@ -38,4 +38,4 @@ Suppressor = "0.2"
 TensorAlgebra = "0.3.2"
 Test = "1"
 TestExtras = "0.3"
-TypeParameterAccessors = "0.3"
+TypeParameterAccessors = "0.4"