"Reading and writing used to be considered separate skills" is a common statement in history writing about the early modern era and 19th century. Which is rather unfortunatel, because it's not actually what people mean. Of course reading and writing are separate skills; they are today, too. People might learn to scratch out the letter forms of their name before they recognize how to read words as words; other people might be reading smoothly before they practice making their own letters mean things.

However, today the assumption is that everyone who learns to read will also learn to (be taught to) write, and vice versa. They are separate skills, but taught to the same people. That is the key shift that occurred over the 19th century in the West. Prior to that, not everyone who learned to read was taught to write.

To discuss this, scholars talk about the uses of literacy: why do people have to learn to read? What will they use it for? Setting aside triumphalist narratives, contemporary sources suggest a few basic pathways encourgaing the teaching of reading: reading for business, daily life, practical affairs; and reading for religious instruction and piety. One of these requires writing to fulfill its important functions, like signing contracts and keeping account books. The other, on the most basic level, does not.

There are two areas where we can see the division most starkly. One is among Swedish women after 1700. Sweden (which at the time includes in its political scope Norway and parts of Finland, /u/commustar) is generally considered the first polity in the West to achieve universal literacy. LONG before any other countries or national churches mobilized, in 1686 the Swedish church mandated that ALL parishioners be aboe to read the Bible for themselves. From church records, it is apparent that this goal was achieved within just a couple of generations. However, looking at marriage and other contracts, a very telling phenomenon appears. Nearly all men are able to sign their own names. However, despite apparently being able to read, many Swedish women simply made a hash mark next to their name signed by someone on their behalf. They could read, but not write enough to sign their name.

The other bug place this is visible is, somewhat surprisingly, among enslaved people in antebellum America. While we absolutely must stress that it was extremely difficult and rare for enslaved people to get an education in the first place, both autobiographies written by former slaves and WPA narrative accounts have plenty of stories from slaves who were allowed, encouraged, or even required to learn to read for religious reasons. Quiet piety was considered good for the soul but also for preserving social order. Writing, on the other hand, was dangerous--it could be used to forge legsl documents certifying freedom or free passes.

For this reason, scholars have sometimes taken to calling the ability to read and write together, "liberating literacy."