simple_constraint_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button 1"
        tools:layout_constraintTop_creator="1" tools:layout_constraintLeft_creator="1"
        app:layout_constraintLeft_toLeftOf="parent" app:layout_constraintTop_toTopOf="parent"/>

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button 2"
        tools:layout_constraintTop_creator="1" app:layout_constraintTop_toBottomOf="@+id/button1"
        tools:layout_constraintLeft_creator="1" app:layout_constraintLeft_toLeftOf="@+id/button1"/>

</android.support.constraint.ConstraintLayout>