How to position below the lowest of two views in ConstraintLayout?

Now it's possible with Barrier class, introduced in constraint-layout v1.1.0.

So here's the solution for your particular case:

<?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"    tools:context="com.example.eugene.test10.MainActivity">    <TextView        android:id="@+id/textView1"        android:layout_width="0dp"        android:layout_height="wrap_content"        android:background="#EEEEEE"        android:text="TEXT_VIEW_1"        app:layout_constraintLeft_toLeftOf="parent"        app:layout_constraintTop_toTopOf="parent"        app:layout_constraintRight_toLeftOf="@+id/textView2"/>    <TextView        android:id="@+id/textView2"        android:layout_width="0dp"        android:layout_height="wrap_content"        android:background="#DDDDDD"        android:text="TEXT_VIEW_2"        android:visibility="gone"        app:layout_constraintLeft_toRightOf="@+id/textView1"        app:layout_constraintRight_toRightOf="parent"        app:layout_constraintTop_toTopOf="parent" />    <android.support.constraint.Barrier        android:id="@+id/labelBarrier"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        app:barrierDirection="bottom"        app:constraint_referenced_ids="textView1,textView2" />    <TextView        android:id="@+id/textView3"        android:layout_width="0dp"        android:layout_height="wrap_content"        android:background="#CCCC00"        android:text="TEXT_VIEW_3"        app:layout_constraintLeft_toLeftOf="parent"        app:layout_constraintRight_toRightOf="parent"        app:layout_constraintTop_toBottomOf="@+id/labelBarrier" /></android.support.constraint.ConstraintLayout>

Here're results for using this layout:

You can refer this step-by-step guide on Codelab https://codelabs.developers.google.com/codelabs/constraint-layout/#10.

<?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">    <TextView        android:id="@+id/header_view_a"        android:layout_width="0dp"        android:layout_height="0dp"        android:background="@android:color/holo_orange_light"        android:gravity="center"        android:text="HeaderViewA "        android:layout_marginBottom="@dimen/sixteenDP"        android:textSize="30sp"        app:layout_constraintBottom_toTopOf="@+id/guideline"        app:layout_constraintStart_toStartOf="parent"        app:layout_constraintTop_toTopOf="@+id/header_view_b"        app:layout_constraintEnd_toStartOf="@+id/header_view_b"        app:layout_constraintVertical_bias="0"        app:layout_constraintWidth_default="wrap" />    <TextView        android:id="@+id/header_view_b"        android:layout_width="wrap_content"        android:layout_height="0dp"        android:background="@android:color/holo_green_light"        android:gravity="center"        android:text="HeaderViewB"        android:textAlignment="center"        android:textSize="30sp"        app:layout_constraintTop_toTopOf="parent"        app:layout_constraintEnd_toEndOf="parent"        app:layout_constraintBottom_toTopOf="@id/guideline"        app:layout_constraintStart_toEndOf="@+id/header_view_a"/>    <android.support.constraint.Guideline        android:id="@+id/guideline"        android:layout_width="0dp"        android:layout_height="wrap_content"        android:orientation="horizontal"        app:layout_constraintGuide_percent="0.4"/>    <TextView        android:id="@+id/big_view"        android:layout_width="0dp"        android:layout_height="0dp"        android:layout_marginBottom="8dp"        android:background="@android:color/holo_blue_dark"        android:gravity="center"        android:text="BigView"        android:textAlignment="center"        android:textSize="@dimen/list_item_height"        app:layout_constraintBottom_toBottomOf="parent"        app:layout_constraintEnd_toEndOf="parent"        app:layout_constraintStart_toStartOf="parent"        app:layout_constraintTop_toTopOf="@+id/guideline"        app:layout_constraintVertical_bias="1.0"/></android.support.constraint.ConstraintLayout>

Watch out for app:layout_constraintWidth_default="wrap" (with width set to 0dp). If set, the widget will have the same size as if using wrap_content, but will be limited by constraints (i.e. it won't expand beyond them) and app:layout_constraintVertical_bias="1.0". Use bias to pull BigView to the bottom of its parent.