When soldiers living in Civil War encampments wanted to cast their vote for Republican incumbent Abraham Lincoln or Democrat George McClellan in the 1864 election, most were able to follow the same process as their parents back home. Lists of registered voters were filled out on the battlefields. De facto election judges and clerks were plucked from the gathered troops. From Kentucky to Vermont, voting rights were extended to those far away from the polls for the first time—though not without significant legal challenges and public skepticism.

More than a century and a half later, voting by mail again took center stage in 2020 as the United States grappled with a pandemic and a presidential election. Here’s what you need