First of all, I would like to say that I set up castles to make a living. While I do a lot of locksmithing, I do not call myself a locksmith because I work exclusively on locks for buildings. That means I do not crack safes or open cars. I own a company that specializes in the sale and installation of door fittings for residential and commercial real estate.
I just came across this question, because I really wondered the same thing with all my expertise. I know the thread is quite old, but I feel like I can contribute to posterity. So, here is my attitude:
There are basically two types of locks on doors: cylinder and mortise lock. Mortise locks consist of a rectangular box that fits into a, well … mortise lock that cuts a joiner into a door. Due to its design, the key is inserted in one side and the latch is attached to the other side. They can not be undone and can only be installed in one direction. There are four types: left, right, left and right reverse.
To understand the types, it is easiest to first understand the cylinder locks. While mortise locks are still manufactured and used, they have fallen out of favor over the years. This is mainly due to the fact that an experienced joiner or a special machine (which no longer manufactures) is required for the installation. A cylinder lock that you probably have at home can easily be installed with a drill and the right bits. Essentially, they just fit in a hole.
Cylinder locks can be reversed as they fit in a round hole instead of a rectangular mortise lock. So unless you have a bent lever or latch, they are all the same. The only time they are "handed over" is when they have an exposed piece that should be shown in a certain way. Even then, they only come in left and right hands. The "bevel" refers to the angle cut on the striker so that the door closes without you first having to turn the knob. Even if a cylinder lock is handed over, you can turn the bevel to the door frame and the closing plate when it is closed. Thus there is no backward slope.
If you are still with me, there are four types of mortise locks, as the key can only be inserted on one side and the bevel of the firing pin can not be turned. There are four types of doors. The hinges can be on the left or right, and the door can open into or out of a room.
For mortise locks, the key has always been inserted vertically and turned away from the frame for locking in the direction of the frame and for unlocking. Likewise, the thumb lock works on the inside. Therefore, it makes sense to install a cylinder lock in the same way as locks have been working this way for at least a century.
However, there are some nuances in cylinder locks that prevent this from becoming a universal rule. The actual locking mechanism of a cylinder lock consists of a flat piece of metal that connects the inside and outside of the lock and passes through the latch that holds the door closed. This is called a "tailpiece".
When the right key is inserted into the lock, you can turn the tailpiece 90 degrees to the left or right from the outside. The tailpiece can be freely rotated by the bolt without a key. If you only hold the locking mechanism, you would see that the tailpiece is a total of 180 freely rotatable. That's why there is this question at all. You can install almost any cylinder lock in the unlocked position with the thumb latch in the vertical OR horizontal position.
So, here's what I know for sure. Some manufacturers design their locks to work in one way or another. As far as I know, this is due to the fact that the lock is stronger when turning to the stop. If you think about it and remember that the tailpiece rotates 180 degrees, the pins of the lock are cocked when it is turned to the left or right, but not when it is in the middle (vertical position).
I know this is confusing, but some locks are designed to be live in either the locked or unlocked position. This depends on the manufacturer and model. I will say that if the direction is important, it is always noted in the instructions. So check them first and follow them.
To make things even more obscure, some manufacturers make locks where the bolt sits at a 45-degree angle. All cylinder locks only need to rotate 90 degrees to operate. Therefore, these special locks tend to lock when locked to the frame and when unlocking. However, they never rest in a vertical or horizontal position. Schlage makes many models in this style.
Recently I also saw cylinder locks with labels on the front panels. Some have an arrow or mark for the locked position. Others have actually locked the words locked and unlocked in the record. To complicate matters, I have seen these labeled faceplates with the locked position vertically, horizontally, and in between.
TLDR: All locks are different. Follow the manufacturer's instructions. If the direction is not specified, install the lock so that it is unlocked when the thumb latch is vertical and locked when it is horizontal. The latch or key locks when it is turned to the frame (strike plate) and unlocks when it is turned away. Whether it is clockwise or counterclockwise depends on whether it is a left or a right door.