Hanging antique white oval mirrors portrait is still the most popular option, but you can easily place them sideways, too.
In fact, this quirkier alternative comes with a few benefits. For example:
it helps create an illusion of a wider room, handy in smaller settings
it also makes it easier to reflect the light of candles and chandeliers
horizontally-set oval mirrors work better in longer rooms with a slightly lower ceiling, too
finally, they can easily become an interesting focal point when you place them above a long element like a mantelpiece