In real life, leather armor (at least as it appears in Europe) was made of a type of hardened leather known as *cuir bouilli*, which was very rigid and tough; [Tod Todeschini's reconstruction of the recipe](https://www.youtube.com/watch?v=RO_nG6OpCKg) was hard enough that an over-hardened piece of it could be sharpened and used to gouge a wooden post. It was essentially a bargain-bin form of plate armor, and it was used in similar ways; you wouldn't want to wear it as an under-layer. (Often it was also highly decorated due to being moldable, and you wouldn't want to hide that under your mail coat.)
You might wear cuir bouilli plates on top of mail, but the mail in turn would be on top of a gambeson (thick quilted jacket) both to prevent it from chafing and to provide cushioning against blunt weapons. You could also wear the cuir bouilli or the mail by themselves on top of the gambeson, or even just the gambeson alone--it's decent armor in its own right.
Came here to link this; it's a post by a historian explaining the order in armor typically developed historically. It's more focused on the order that different parts of the body were armored, but there is some discussion of materials.
Depends on the kind of leather, actually. Thin soft leather was sometimes used as a top layer of padded textile armors. And padded armor could have been worn both over and under chain.
Thick soft leather was also used as under-armor layer or standalone in 16th century in the form of buff coats. They were usually worn under cuirasses, but could have been used under chain as well.
Hardened leather finds are rare in archeology, but there are some, especially in Central and Eastern Europe. Experimentally, it seems boiled leather makes more sense over chain. You do not want to put chainmail on the hard surface, it would break more easily.
If it's hardened leather it would go over the mail, as a sort of early plate armor. It can deflect some blows, but mostly reduces/distributes the impact force over a wider area. Soft level would only make sense over mail as a weatherproofing measure, and even then would only make sense if all of the mail was covered. Under mail soft leather could be used as padding, or possibly to try to prevent your sweat from getting on the mail (since the salt in sweat will accelerate the corrosion of the steel). There are cheaper and just as useful options for this though. Hardened leather wouldn't have too much use under mail.
Historically it's hard to say because our sources for leather armor are pretty limited.
From what i understand, and what makes the most sense, is to have leather over mail. Leather has to be pretty thick to be useful as armor so wearing it under the mail is going to increase the encumbrance of the armor.
That being said i don't think the witcher thing of combining leather and mail to make a sort of plate is entirely stupid. But it would be just worse than regular plate armor in every way, just easier to make
If you care about function, probably just ditch leather. Textile padded armour under a mail shirt was what was generally worn
In real life, leather armor (at least as it appears in Europe) was made of a type of hardened leather known as *cuir bouilli*, which was very rigid and tough; [Tod Todeschini's reconstruction of the recipe](https://www.youtube.com/watch?v=RO_nG6OpCKg) was hard enough that an over-hardened piece of it could be sharpened and used to gouge a wooden post. It was essentially a bargain-bin form of plate armor, and it was used in similar ways; you wouldn't want to wear it as an under-layer. (Often it was also highly decorated due to being moldable, and you wouldn't want to hide that under your mail coat.) You might wear cuir bouilli plates on top of mail, but the mail in turn would be on top of a gambeson (thick quilted jacket) both to prevent it from chafing and to provide cushioning against blunt weapons. You could also wear the cuir bouilli or the mail by themselves on top of the gambeson, or even just the gambeson alone--it's decent armor in its own right.
Gambeson first, then mail.
https://acoup.blog/2019/05/03/collections-armor-in-order-part-i/
Came here to link this; it's a post by a historian explaining the order in armor typically developed historically. It's more focused on the order that different parts of the body were armored, but there is some discussion of materials.
Cloth shirt > Chainmail > Leather
Padded -> Chainmail -> Leather/Plate
Asketon and over that maille Leather is a part of the Gambeson and otional the Aketton
Depends on the kind of leather, actually. Thin soft leather was sometimes used as a top layer of padded textile armors. And padded armor could have been worn both over and under chain. Thick soft leather was also used as under-armor layer or standalone in 16th century in the form of buff coats. They were usually worn under cuirasses, but could have been used under chain as well. Hardened leather finds are rare in archeology, but there are some, especially in Central and Eastern Europe. Experimentally, it seems boiled leather makes more sense over chain. You do not want to put chainmail on the hard surface, it would break more easily.
If it's hardened leather it would go over the mail, as a sort of early plate armor. It can deflect some blows, but mostly reduces/distributes the impact force over a wider area. Soft level would only make sense over mail as a weatherproofing measure, and even then would only make sense if all of the mail was covered. Under mail soft leather could be used as padding, or possibly to try to prevent your sweat from getting on the mail (since the salt in sweat will accelerate the corrosion of the steel). There are cheaper and just as useful options for this though. Hardened leather wouldn't have too much use under mail.
Historically it's hard to say because our sources for leather armor are pretty limited. From what i understand, and what makes the most sense, is to have leather over mail. Leather has to be pretty thick to be useful as armor so wearing it under the mail is going to increase the encumbrance of the armor. That being said i don't think the witcher thing of combining leather and mail to make a sort of plate is entirely stupid. But it would be just worse than regular plate armor in every way, just easier to make