Take 'rise' and add -er: the one that goes up. Sometimes the thing itself travels upward — a pipe climbing through a building, a person getting out of bed. Sometimes it stands still and takes something else up with it — the upright face of a , a block under a screen, a set of steps under a back row of singers. Either way, the picture points in one direction, and that direction is up.
From the word’s history
The supplied Wiktionary entry gives riser as Middle English ryser, risere, equivalent to rise + -er. The modern senses are still just that: the one that rises, or the one that raises.
Wiktionary 記的是中古英文 ryser、risere，等於 rise 加 -er。今天的各種用法其實還停在這個字面上：往上的那一個，或是把東西弄上去的那一個。
Wiktionary entry for 'riser' as supplied in this evidence packet: 'From Middle English ryser, risere, equivalent to rise + -er.'
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