After states answered Lincoln's April 15, 1861, ninety-day call for 75,000 volunteer soldiers, most Union states' regiments and batteries became known as "Volunteers" to distinguish between state and regular army units.
Efter at staterne havde reageret på Lincolns appel den 15. april om 75.000 frivillige i løbet af 90 dage, blev de fleste staters militsregimenter og artilleribatterier kendt som "frivillige", for at man kunne adskille dem fra den regulære hær.