diff --git a/lib/hd/private.js b/lib/hd/private.js
index 935c41a725..c908fc6708 100644
--- a/lib/hd/private.js
+++ b/lib/hd/private.js
@@ -82,10 +82,7 @@ class HDPrivateKey {
     assert(Buffer.isBuffer(options.privateKey));
 
     if (options.purpose) {
-      assert(
-        ['x','y','z'].indexOf(options.purpose) !== -1,
-        'Bad purpose'
-      );
+      assert(common.purposes[options.purpose] !== undefined, 'Bad purpose');
       this.purpose = options.purpose;
     } else {
       this.purpose = 'x';
diff --git a/lib/hd/public.js b/lib/hd/public.js
index b11a1beccc..5b7275ead3 100644
--- a/lib/hd/public.js
+++ b/lib/hd/public.js
@@ -71,10 +71,7 @@ class HDPublicKey {
     assert(Buffer.isBuffer(options.publicKey));
 
     if (options.purpose) {
-      assert(
-        ['x','y','z'].indexOf(options.purpose) !== -1,
-        'Bad purpose'
-      );
+      assert(common.purposes[options.purpose] !== undefined, 'Bad purpose');
       this.purpose = options.purpose;
     } else {
       this.purpose = 'x';
diff --git a/lib/wallet/account.js b/lib/wallet/account.js
index b730047a2e..9027f4e404 100644
--- a/lib/wallet/account.js
+++ b/lib/wallet/account.js
@@ -103,12 +103,10 @@ class Account {
     }
 
     if (options.purpose != null) {
-      assert(typeof options.purpose === 'string');
-      assert(
-        ['x','y','z'].indexOf(options.purpose) !== -1,
-        'Bad purpose'
-      );
+      assert(common.purposes[options.purpose] !== undefined, 'Bad purpose');
       this.purpose = options.purpose;
+    } else {
+      this.purpose = 'x';
     }
 
     if (this.purpose === 'y' || this.purpose === 'z')
@@ -481,19 +479,11 @@ class Account {
     let key;
     if (master && master.key && !this.watchOnly) {
       const coinType = this.network.keyPrefix.coinType;
-
-      switch (this.purpose) {
-        case 'x':
-          key = master.key.deriveAccount(44, coinType, this.accountIndex);
-          break;
-        case 'y':
-          key = master.key.deriveAccount(49, coinType, this.accountIndex);
-          break;
-        case 'z':
-          key = master.key.deriveAccount(84, coinType, this.accountIndex);
-          break;
-      }
-
+      key = master.key.deriveAccount(
+        common.purposePath[this.purpose],
+        coinType,
+        this.accountIndex
+      );
       key = key.derive(branch).derive(index);
     } else {
       key = this.accountKey.derive(branch).derive(index);
@@ -889,7 +879,7 @@ class Account {
     if (this.witness)
       flags |= 2;
 
-    const purposeBits = Account.purposes[this.purpose];
+    const purposeBits = common.purposes[this.purpose];
     flags |= (purposeBits << 6);
 
     bw.writeU8(flags);
@@ -921,7 +911,7 @@ class Account {
     const flags = br.readU8();
 
     const purposeBits = flags >> 6;
-    this.purpose = Account.purposesByVal[purposeBits];
+    this.purpose = common.purposesByVal[purposeBits];
 
     this.initialized = (flags & 1) !== 0;
     this.witness = (flags & 2) !== 0;
@@ -988,26 +978,6 @@ Account.typesByVal = [
   'MULTISIG'
 ];
 
-/**
- * Account purposes.
- * @enum {Number}
- * @default
- */
-
-Account.purposes = {
-  x: 0,
-  y: 1,
-  z: 2
-};
-
-/**
- * Account purposes by value.
- * @enum {Number}
- * @default
- */
-
-Account.purposesByVal = ['x', 'y', 'z'];
-
 /**
  * Default address lookahead.
  * @const {Number}
diff --git a/lib/wallet/common.js b/lib/wallet/common.js
index 01a7c209a1..12a587fc31 100644
--- a/lib/wallet/common.js
+++ b/lib/wallet/common.js
@@ -143,3 +143,39 @@ common.sortDeps = function sortDeps(txs) {
 
   return result;
 };
+
+/**
+ * Purposes from extended key prefix bytes.
+ * @enum {Number}
+ * @default
+ */
+
+common.purposes = {
+  x: 0,
+  y: 1,
+  z: 2
+};
+
+/**
+ * Purposes by value.
+ * @enum {Number}
+ * @default
+ */
+
+common.purposesByVal = [
+  'x',
+  'y',
+  'z'
+];
+
+/**
+ * BIP44 "purpose" values by extended key prefix bytes
+ * @enum {Number}
+ * @default
+ */
+
+common.purposePath = {
+  x: 44,
+  y: 49,
+  z: 84
+}