一
Means One; a, an; per
[1]
 
Memory hook Resembles a single line drawn left to right.
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[1] B-2 Spirit Contrails by user brentdanley from http://www.flickr.com/photos/brentdanley/287212038/, flipped horizondal, detail, under license: Attribution-Noncommercial-Share Alike 2.0 Generic