A turnaround is two bars whose only job is to get you back to bar one. It is the smallest complete unit in this music, and it is the reason the last two bars of a chorus never simply sit there: the form has to be handed back, and handing it back is a harmonic job with several accepted solutions.
Four of them, and they are alternatives
The four declared here are not a sequence. Nobody plays them one after another; a group picks one, often without discussing it, and everybody hears which one it was by the second chord. That is why nothing on this page reports a chorus length for them — a set of choices has no length, and the engine refuses to invent one.
The plain one goes tonic, submediant, supertonic, dominant, and is the version to know first. The second replaces the submediant minor with a dominant built on the same root, which is the single most common alteration in the repertoire and changes the character of the two bars completely — the chord is now pulling somewhere instead of sitting. The third gives up the tonic entirely and starts on the mediant, which keeps two of the tonic's three notes and lets the bass move while the top stays put. The fourth climbs: tonic, supertonic, mediant, then a dominant on the sixth degree.
The rising one earns its place twice
That fourth turnaround is the only declared passage on this site where two neighbouring chords sit a whole tone apart — twice, in fact — which makes it the one place the diminished passing rule has anything to do. That is not a happy accident; it is why it is declared. A rule with no worked example anywhere is a claim rather than an engine, and the engine's job is to be checkable.
In any key
They open in C here because the two bars are short enough to hear in your head, and C is the key most readers can do that in without transposing first. Every other key is a click away, and the substitutions below are recomputed with it.