Yes, as mentioned, the Sonos Five/Play:5 speakers do work horizontally, as well as vertically, as a stereo pair. The support documentation says this:

Speaker orientation guidelines

You can place a Sonos Five, Play:5 (gen2) or Play:3 in either a horizontal or vertical orientation.

A horizontal speaker will output stereo audio over a wide soundstage while a vertical speaker will output mono audio/single audio channel. When placing a single speaker, Sonos recommend using a horizontal orientation. 

When either horizontal or vertical, make sure all four rubber feet are sitting completely on the flat surface. 

I will just personally add to the above with this…

A single vertical speaker will not play in stereo (it’s mono audio only), whereas a single horizontal will have a left/right channel. Two speakers paired in stereo will play in stereo regardless of orientation.

If using the Fives as a stereo pair, then orient vertically with tweeters on the inside for a sharper stereo image, or put the tweeters on the outside for a slightly wider, but possibly more diffuse soundstage.

Hope that helps.

@Ken_Griffiths If you had a decent area and two Play:5s would having them vertical with a tighter sound stage have better sound vs them horizontal, or would it sound pretty similar?

The answer to this is so dependent on where the speakers are placed in the space, and so easy to check which way sounds better, that it is best to just try out all ways and let your preference decide what is best.

I have a 22’ x 14’ room and I have chosen to put my two Fives vertical with their tweeters on the inside - I have ‘em bonded with a Sub (gen3) and they are approx at head height (when seated) on the opposite longer wall to where I sit - approx 4ft from each corner.

The Sub, placed on the floor is roughly in the middle of the two on the same wall, but also in a slight corner as there is a pillar  that juts out, where the room was extended. I’ve just added a quick sketch to assist with the description. 

Here’s a slightly interesting point I have with my own setup. The left hand ‘wall’ as you look at my sketch in the post above is actually a large double glazed patio door/window, which has lined curtains that cover the entire wall, which we open in the day and close in the evening. That means the room acoustics change depending on whether those heavy curtains are open or closed - it’s one reason why I would like to be able to save Trueplay settings for the setup. One for when the curtains are open and one when they are drawn.

My Wife also likes to put a plant in front of the Sub, which gets moved (lots) on a regular basis, when she steps out the room (but that’s another story).🤫

For some listeners there is a significant difference between horizontal and vertical, for others there is not much difference. And, the room layout may make some difference. Only you know what sounds “best” (for you). It’s easy, and cheap, to experiment.

Probably make sense for now to run Trueplay with the curtains half open. And you should be able to see how much their state matters by toggling Trueplay on/off, with music playing with curtains fully open and fully closed, and seeing if the differences differ!

I also find that True play has the most impact on the sound of the Sub - and that depends more on where the Sub is placed in the room. LF waves pass through concrete, so curtains won’t affect how they  propagate in the room. HF on the other hand will be audibly affected.