Main toolbar > Align (on Align flyout) > Select object.
The Align dialog enables you to align the current selection to a target selection. The name of the target object appears in the title bar of the Align dialog. When performing sub-object alignment, the title bar of the Align dialog reads "Align Sub-Object Selection."
You can align the position and orientation of the bounding box of the source object to the bounding box of a target object.
You can use the Align tool with any selection that can be transformed. If an axis tripod is displayed, you can align the tripod (and the geometry it represents) to any other object in the scene.
When performing sub-object alignment, the Current Object options and the Match Scale check boxes are disabled. If you plan to align orientation for sub-objects, first switch to Local transform mode on the Main toolbar so that the axis tripod is properly aligned with your sub-object selection.
To align an object with a point object:
Create a point helper object and position it at a target location in your scene. Rotate it as necessary to adjust final orientation.
Select a source object.
Click Align on the Main toolbar, or choose Tools > Align.
The Align cursor appears attached to a crosshair.
Move the cursor over the point object and click.
The Align Selection dialog appears. If necessary, move the dialog out of the way so you can see the active viewport.
In the Align Position group, select X Position.
The selected source object shifts to align with the X-axis of the point object.
Select Y Position and Z Position.
The source object moves so its center is at the point object.
Select X, Y, and Z Axis in the Align Orientation group to reorient the object to match the coordinates of the point.
To align objects by position and orientation:
Select a source object (the object to move into alignment with the target object).
Click Align on the Main toolbar, or choose Tools > Align.
The Align cursor appears attached to a crosshair.
Move the cursor over the target object and click.
The Align Selection dialog appears. By default, all options in the dialog are turned off.
Under Current Object and Target Object, click Minimum, Center, Pivot Point, or Maximum.
These settings establish the points on each object that become the alignment centers.
Begin alignment by clicking X, Y, and Z Positions.
The source object moves in relation to the target object, along the axes of the reference coordinate system. Setting all three moves the objects as close as possible, given the Current and Target Object Settings.
In the Apply Orientation group, click X, Y, or Z Axis.
The source object realigns accordingly. If the objects share an orientation, that axis has no effect. Once two axes are aligned in orientation, the third is automatic.
To align a gizmo to another object:
Display the gizmo level of the Sub-Object selection.
Click Align on the Main toolbar.
Click to select a target object in the viewport. (You can select the same object containing the gizmo to align the gizmo to a part of its own object.)
Use the available settings in the Align dialog to adjust the transformation of the gizmo.
To align a sub-object selection of geometry to another object:
Do one of the following:
Convert the object to an editable mesh, and then perform the sub-object selection at any level.
Apply a Mesh Select modifier, followed by an Xform modifier. (The Mesh Select modifier by itself doesn't allow transforms.)
Click Align, and then select a target object.
Use the settings in the Align dialog to perform the alignment.
Align Position (Screen)
X, Y, Z Position: Specifies on which axis or axes to perform the alignment. Selecting all three boxes centers the current object on the target object.
Current Object/Target Object
Specifies which point on the objects' bounding boxes to use for the alignment. You can choose different points for the current object and the target object. For example, you can align the current object's pivot point with the center of the target object.
Minimum: Aligns the closest point of the object's bounding box with the chosen point on the other object.
Center: Aligns the center of the object's bounding box with the chosen point on the other object.
Pivot Point: Aligns the object's pivot point with the chosen point on the other object.
Maximum: Aligns the farthest point of the object's bounding box with the chosen point on the other object.
Align Orientation (Local)
Lets you match the orientation of the local coordinate systems between the two objects on any or all three axes.
This option is independent of the position alignment settings. You can leave the position settings alone and select all three Rotation check boxes to rotate the current object to match the orientation of the target object.
Position alignment uses world coordinates, while orientation alignment uses local coordinates.
Match Scale
Click the X Axis, Y Axis, or Z Axis options to match the scale axis values between the two selected objects. This matches only the scale values you'd see in the Transform Type-In. It will not necessarily cause two objects to be the same size. There will be no change in size if neither of the objects has previously been scaled.