A Newman projection looks down the C2-C3 bond of 3-chlorobutan-2-ol, C2 in front. Front carbon (C2): CH${}_3$ at 12 o'clock, OH at 4 o'clock, H at 8 o'clock. Back carbon (C3): H at 2 o'clock, Cl at 6 o'clock, CH${}_3$ at 10 o'clock. What is the configuration of the compound?

At C2 the priorities are OH > C3 (Cl,C,H) > CH${}_3$ > H. C3 points away from the viewer, and OH (4 o'clock) $\to$ CH${}_3$ (12) $\to$ H (8), i.e. priorities 1 $\to$ 3 $\to$ 4, run counter-clockwise; with priority 2 pointing away this corresponds to S (swapping priorities 2 and 4 would give a clockwise 1-2-3 sequence, R, so the real centre is S). At C3 the priorities are Cl > C2 (O,C,H) > CH${}_3$ > H, and C2 points towards the viewer; Cl (6) $\to$ CH${}_3$ (10) $\to$ H (2) runs clockwise, which with priority 2 towards the viewer is the same as counter-clockwise with it away, so C3 is also S. The compound is (2S,3S); rotation about C2-C3 does not change configuration.