It's time for another Hearts of Iron IV Developer Diary, and this time I'm going to talk a bit more about production; specifically the Production Lines and How Things Get Built. All military equipment is made on Production Lines in HoI IV, so players will need to be familiar with how they work if they want to maximize their war machine.But before we can talk about the production lines themselves, we should cover how Industry has changed in HoI IV.First of all, we have separated industry in Hearts of Iron IV into 3 types:- Used both for "Consumer goods" and building infrastructure and other buildings.– Used for building ships.- Used for production of military equipment such as tanks, weapons and airplanes.This kind of separation allows us to balance different countries' industrial strengths (the capacity to make ships is not the same as the capacity to make luxuries), and gives the player a more interesting selection of targets for strategic bombing.Moving on to the actual means of Production, Factories no longer need Metal/Energy/Rare Materials to run. Requiring the player to gather several different types of resources in order to manage factories did not necessarily add anything interesting to the mix. Being short on any of them had the same effect no matter what you were lacking (your Industrial Capacity would shrink) and it didn't entirely make sense that you couldn't build things like Militia if you didn't have access to Rare Materials.We have simplified the inputs to "Raw materials" which factories use to run. Raw materials act as a limit on your total Industrial Capacity. However that is not the whole Production story. Equipment also has a Strategic Resource cost, without which it takes much longer to produce. Strategic Resources are not accumulated in pools. Instead, they represent the potential flow of resources into your factories. For example if you have 10 Iron you can be building stuff that costs up to 10 Iron at any one time.A Production Line is a standing order for a factory or group of factories to make a certain piece of equipment. Each piece of equipment has an IC (Industrial Capacity) cost and a Resource cost. The IC cost determines how much equipment each factory can produce per week, while the Resource cost determines how many resources are needed for the line to operate at full speed.As a totally made up example: An Advanced Medium Tank may cost 2 IC and require 1 Iron and 1 Tungsten. Each Factory produces 10 IC, so if you assigned 1 Factory to this Production Line, you would produce 5 Advanced Medium Tanks per week. If you assign 10 factories you would get 50 tanks/week. At the same time you would need to have 5 Iron and 5 Tungsten in the first case, and 50 Iron and 50 Tungsten in the second. (Again, these numbers are all purely made up, focus on the idea and not the values here.) You can only assign up to 15 Factories to any given Production Line, so you won't be able to build, say, a Battleship in one week by assigning 100 Factories to build it.Also, do note that simply building Equipment is not the same as training and equipping a unit, but we'll cover that in a future dev diary.Production Lines also have an Efficiency value which affects how much value you get out of your IC at a factory. Your efficiency starts out fairly low but increases as items are produced - slowly at first to represent the retooling of the factories, then it begins to increase at a linear rate until tapering off after a certain value (an S curve). You can change what a Production line produces, of course, and this normally means all your Efficiency is lost, however there are some exceptions. If you change to a modified version of the same equipment (for example, the same tank but with a larger gun) you keep most of your Efficiency. If you switch to another variant of the same chassis (e.g. you switch from Pz IIIs to StuG IIIs) you keep half your Efficiency. And if you switch within the same family (e.g. Basic Medium Tank to Improved Medium Tank) you keep a small part of your Efficiency.Efficiency means that you will be able to produce more once your factories are humming along. So long as you can keep your workers on task and supplied with what they need, you will be able to have assembly line production that properly reflects the might of an economy dedicated to the war effort.Our larger hope is that Production Lines and efficiency will offer players some interesting choices when it comes to deciding what to build. Should you go for a large number of weapons you can already churn out, or take a short term hit on production in favor of making a smaller number of higher quality ones? Sure, your new T-43 tank is better than the T-34, but is it really enough of an improvement to lose much of your Production Line's Efficiency when you switch over? Your PzIII tank may be obsolete, but perhaps instead of canceling their production entirely you could convert the Line to make Tank Destroyers or Self-Propelled Artillery on the PzIII chassis. Preserving efficiency in some of your factories could lead to a more diverse and interesting combination of units, and allow you to discover some parts of the game you might have ignored if you just constantly upgraded.