BEGIN TRANSACTION within TRY - CATCH or vice versa

Which of the following connotes the correct coding block structure?

BEGIN TRANSACTION
BEGIN TRY
    --================================================
    -- Add Your Code Here
    --================================================
END TRY
BEGIN CATCH
 ROLLBACK
END CATCH
COMMIT

or

BEGIN TRY
    BEGIN TRANSACTION
    --================================================
    -- Add Your Code Here
    --================================================
    COMMIT
END TRY
BEGIN CATCH
 ROLLBACK
END CATCH