It is possible to arrange regions in a playlist (track) so that they overlap - one starts before another finishes, for example. Because of this, its important that there is a clear and understandable rule for what you will hear when playing back these kinds of region arrangements. Every region in a playlist is assigned to a layer. There can only ever be one region on a given layer, although rearranging the playlist (track) may change which region is on which layer. At any given point along the timeline, you will hear the uppermost region at that point. Of course, nothing in digital audio is ever quite that simple, and so there are some complications:

Whenever two regions overlap, there is the potential for a crossfade between them. If the crossfade is not muted, then you will hear the contents of the crossfade during the overlap, not just the uppermost region.

With a nod to image manipulation programs, Ardour allows you to make regions transparent. By default, all regions are created opaque, which means that when they are playing, no region below them are audible. However, if you change the region to be transparent, the region will be audible together with any regions below it. This capability should probably not be abused; if you really want to mix sounds together in this way, they should probably be on their own tracks. Occasionally though, this can be a useful trick. To change the opacity of a region, popup the region's editor, accessible by context clicking on the region. Then click on the "opaque" button, turning it on or off as desired.

There are two main decisions to be made with regard to how a playlist should be layered: Given overlapping regions, which order should they be layered in? When should layering be changed?

Ardour provides three-and-a-half ways to decide on the order in which regions are layered. The most basic choice is: Most recently added regions are higher Regions which are later in time will be on higher layers. Most recently added or edited regions are higher Regions which were more recently edited or added to the playlist will be on higher layers. Later regions are higher Regions which were more recently added to the playlist will be on higher layers.

A new session has the layering style set to "Most recently edited or added regions are higher". To change the layering style, open the Session Properties dialogue and choose your layering style from the "Misc" page. Changing the layering style only affects future edits to the playlist; the existing layering of all playlists is preserved when changing the layering mode.

If you want a particular region to be the uppermost when the current layering style has put it on a lower layer, context click on the region. Select the region from the menu that pops up, and in the submenu that appears, choose one of "Move region to lowest layer" or "Move region to upper layer". The layering for the playlist will be recalculated to ensure that the region is on the layer you chose. Note: these operations only have any effect if using one of the "Most recently .." layer models. You can see the precise layer a region is assigned in the popup region editor, accessible by context clicking on the region. At one time, Ardour offered more explicit control over the layering, allowing you to move regions up or down to specific layers. This was found to be problematic, confusing, and generally rendered unnecessary by both of the layering styles the program now offers.