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"distance between two objects," Old English span "distance between the thumb and little finger of an extended hand," probably related to Middle Dutch spannen "to join, fasten" (see span (n.2)).
The Germanic word was borrowed into Medieval Latin as spannus, hence Italian spanna, Old French espanne, French empan. As a measure of length, roughly nine inches. Meaning "length of time" first attested 1590s; that of "space between abutments of an arch, etc." is from 1725. Meaning "maximum lateral dimension of an aircraft" is first recorded 1909. Attention span is recorded from 1922.
"two animals driven together," 1769, from Dutch span, from spannen "to stretch or yoke," from Middle Dutch spannen, cognate with Old English spannen "to join" (see span (v.)).
Old English spannen "to clasp, fasten, stretch, span," from Proto-Germanic *spanwanan (cf. Old Norse spenna, Old Frisian spanna, Middle Dutch spannen, Old High German spannan, German spannen), from PIE root *(s)pen- "to draw, stretch, spin" (cf. Latin pendere "to hang, to cause to hang," pondus "weight" (the weight of a thing measured by how much it stretches a cord), pensare "to weigh, consider;" Greek ponein "to toil;" Lithuanian spendziu "lay a snare;" Old Church Slavonic peti "stretch, strain," pato "fetter," pina "I span;" Old English spinnan "to spin;" for other cognates, see spin (v.)). The meaning "to encircle with the hand(s)" is from 1781; in the sense of "to form an arch over (something)" it is first recorded 1630s.