dou·ble–space 
/ˌdʌbəlˈspeɪs/ verb
dou·ble–spac·es; dou·ble–spaced; dou·ble–spac·ing /ˌdʌbəlˈspeɪs/ verb : to write or type (a paper, letter, etc.) so that each line of words is followed by a line without words [+ obj] — often used as (be) double-spaced ▪ All essays must be typed and double-spaced. [no obj] ▪ Be sure to double-space. It makes reading much easier.
