diff --git a/lib/sdk.test.js b/lib/sdk.test.js
index fc56063..df33db7 100644
--- a/lib/sdk.test.js
+++ b/lib/sdk.test.js
@@ -21,17 +21,29 @@ test("eid ignores case", () => {
   expect(eid).toEqual(OptableSDK.eid(var4));
 });
 
-test("cid is correct", () => {
-  const expected = "c:FooBarBAZ-01234#98765.!!!";
+test("cid rejects non-string id", () => {
+  expect(() => OptableSDK.cid(1)).toThrow();
+});
 
-  expect(expected).toEqual(OptableSDK.cid("FooBarBAZ-01234#98765.!!!"));
-  expect(expected).toEqual(OptableSDK.cid(" FooBarBAZ-01234#98765.!!!"));
-  expect(expected).toEqual(OptableSDK.cid("FooBarBAZ-01234#98765.!!!  "));
-  expect(expected).toEqual(OptableSDK.cid("  FooBarBAZ-01234#98765.!!!  "));
+test("cid prefixes with c: by default", () => {
+  expect(OptableSDK.cid("abc")).toEqual("c:abc");
 });
 
-test("cid is case sensitive", () => {
-  const unexpected = "c:FooBarBAZ-01234#98765.!!!";
+test("cid accepts a custom variant", () => {
+  expect(OptableSDK.cid("abc", 0)).toEqual("c:abc");
+  for (let i = 1; i < 10; i++) {
+    expect(OptableSDK.cid("abc", i)).toEqual(`c${i}:abc`);
+  }
+
+  expect(() => OptableSDK.cid("abc", -1)).toThrow();
+  expect(() => OptableSDK.cid("abc", 10)).toThrow();
+  expect(() => OptableSDK.cid("abc", "1")).toThrow();
+});
+
+test("cid trim spaces", () => {
+  expect(OptableSDK.cid(" \n abc\t")).toEqual("c:abc");
+});
 
-  expect(OptableSDK.cid("foobarBAZ-01234#98765.!!!")).not.toEqual(unexpected);
+test("cid preserve case", () => {
+  expect(OptableSDK.cid("ABCD")).toEqual("c:ABCD");
 });
diff --git a/lib/sdk.ts b/lib/sdk.ts
index c1f20c1..927c379 100644
--- a/lib/sdk.ts
+++ b/lib/sdk.ts
@@ -99,8 +99,22 @@ class OptableSDK {
     return email ? "e:" + sha256.hex(email.toLowerCase().trim()) : "";
   }
 
-  static cid(ppid: string): string {
-    return ppid ? "c:" + ppid.trim() : "";
+  static cid(ppid: string, variant: number = 0): string {
+    let prefix = "c:";
+
+    if (typeof ppid !== "string") {
+      throw new Error("Invalid ppid");
+    }
+
+    if (typeof variant !== "number" || isNaN(variant) || variant < 0 || variant > 9) {
+      throw new Error("Invalid variant");
+    }
+
+    if (variant > 0) {
+      prefix = `c${variant}:`;
+    }
+
+    return ppid ? prefix + ppid.trim() : "";
   }
 
   static TargetingKeyValues(tdata: TargetingResponse): TargetingKeyValues {