For the armhole of a sweater, it doesn't matter how you decrease. It's just simplest to knit 2 together on each edge, or purl 2 together on each edge.
See, the decrease won't show at all once you sew up the sleeves in the armhole. Or, if you're making a vest, once you put the ribbing around the armhole edge to finish it off.
So just make it easy on yourself. Hey, if slip 1, knit 1, psso is your preferred method of decreasing, gofer it.
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