diff --git a/lib/rex/book/limit_volume_tracker.rb b/lib/rex/book/limit_volume_tracker.rb
index be61cb8..e4e8cfe 100644
--- a/lib/rex/book/limit_volume_tracker.rb
+++ b/lib/rex/book/limit_volume_tracker.rb
@@ -6,29 +6,32 @@ module Rex
       attr_reader :volumes
 
       def initialize
-        @volumes = Hash.new { 0 }
+        @volumes = {
+          true => Hash.new { 0 },
+          false => Hash.new { 0 }
+        }
       end
 
       def add_order(order)
-        volumes[[order.is_buy, order.price]] += order.remaining_quantity
+        volumes[order.is_buy][order.price] += order.remaining_quantity
 
         [
           LimitVolumeChange.new(
             side(order),
             order.price,
-            volumes[[order.is_buy, order.price]]
+            volumes[order.is_buy][order.price]
           )
         ]
       end
 
       def remove_order(order)
-        volumes[[order.is_buy, order.price]] -= order.remaining_quantity
+        volumes[order.is_buy][order.price] -= order.remaining_quantity
 
         [
           LimitVolumeChange.new(
             side(order),
             order.price,
-            volumes[[order.is_buy, order.price]]
+            volumes[order.is_buy][order.price]
           )
         ]
       end
@@ -37,18 +40,18 @@ module Rex
         buy_order = trade.buy_order
         sell_order = trade.sell_order
 
-        volumes[[buy_order.is_buy, buy_order.price]] -= trade.quantity
-        volumes[[sell_order.is_buy, sell_order.price]] -= trade.quantity
+        volumes[buy_order.is_buy][buy_order.price] -= trade.quantity
+        volumes[sell_order.is_buy][sell_order.price] -= trade.quantity
         [
           LimitVolumeChange.new(
             :buy,
             buy_order.price,
-            volumes[[buy_order.is_buy, buy_order.price]]
+            volumes[buy_order.is_buy][buy_order.price]
           ),
           LimitVolumeChange.new(
             :sell,
             sell_order.price,
-            volumes[[sell_order.is_buy, sell_order.price]]
+            volumes[sell_order.is_buy][sell_order.price]
           )
         ]
       end
diff --git a/spec/book/limit_volume_tracker_spec.rb b/spec/book/limit_volume_tracker_spec.rb
index ae2e20a..87c5e87 100644
--- a/spec/book/limit_volume_tracker_spec.rb
+++ b/spec/book/limit_volume_tracker_spec.rb
@@ -8,7 +8,7 @@ RSpec.describe Rex::Book::LimitVolumeTracker do
   describe "#add_order" do
     it "adds the quantity to the limit volume" do
       expect { instance.add_order(sell_order) }.to(
-        change { instance.volumes[[false, 100]] }.to(250)
+        change { instance.volumes[false][100] }.to(250)
       )
     end
 
@@ -26,7 +26,7 @@ RSpec.describe Rex::Book::LimitVolumeTracker do
 
     it "removes the quantity from the limit volume" do
       expect { instance.remove_order(sell_order) }.to(
-        change { instance.volumes[[false, 100]] }.from(250).to(0)
+        change { instance.volumes[false][100] }.from(250).to(0)
       )
     end
 
@@ -54,9 +54,9 @@ RSpec.describe Rex::Book::LimitVolumeTracker do
 
     it "adjusts the volumes of the involved limits" do
       expect { instance.process_trade(trade) }.to(
-        change { instance.volumes[[false, 100]] }.from(250).to(50)
+        change { instance.volumes[false][100] }.from(250).to(50)
         .and(
-          change { instance.volumes[[true, 100]] }.from(200).to(0)
+          change { instance.volumes[true][100] }.from(200).to(0)
         )
       )
     end