As someone who is new to Second Life I am constantly learning how to do things as basic as sitting in chairs, unpacking boxed items and understanding why some scripts work in some sims and not in others. To assist me in my Second Life adventures I’ve found many helpful people who have given me the benefit of their advice and, on occasions when there is no one to offer support, there is a surprising amount of information to be found on websites such as Second Life wiki, Youtube tutorials and the Second Life Forums. So rather than rely entirely on my own limited experience to explain how alpha layers work, I will offer quotes from more experienced SL residents to describe the process of using multiple mesh and alpha layers.
For those who are newer to Second Life than myself, I will give a brief explanation of what mesh and alpha layers are. There are three types of clothes in second life, the first is usually called “system clothes” which are represented in your inventory as shirt and pants icons. System clothes appear to be "painted" onto your avatar and don’t have the same realistic quality as the other two types. The second type of clothing is called “Prim” and is represented in your inventory as a yellow cube. Prim clothes are usually made from multiple prims linked together which sometimes appears more realistic than system clothes but, because it is composed of several parts, it can sometimes cause lag. The third type of clothing is called “Mesh” and is also represented in your inventory as a cube.
Unlike system clothes Mesh is more realistic in the way it fits on your avatar and, unlike prim clothing, mesh does not generate lag, so it would appear that Mesh is the best choice of the three. However when mesh clothes are worn, sometimes, it does not fit perfectly and parts of your avatar will show through the clothes. So, if your avatar has a large posterior and you put on a pair of mesh skinny jeans, your backside will show through the jeans. One way to deal with this problem is to readjust your avatar's shape to fit the clothes however, for mesh clothing designers, a more convenient solution was to create an alpha layer, which makes parts of your avatar invisible.
Usually when a mesh outfit is purchased, an alpha layer that fits the outfit is included, however this is not always the case, especially with gift and discount items. Problems with alpha layers also occur when clothes purchased separately are mixed together to create a unique ensemble. This is when it becomes necessary to use multiple alpha layers and you will need to know how to “add” instead of wear.
When Second Life resident Daika Andel posted the question "How do i wear alpha layers together” in the SL Forums back in November 2012, Charlotte Caxton replied; "When you want to wear more than one of something, right click and choose add instead of wear. Some older viewers do not support this feature. You can wear up to five layers at once of each layer, ie 5 shirt layers, and 5 tattoo layers and 5 alpha layers etc. Multiple layers of some tattoo layers occasionally cause issues sometimes for some people.”
When posed a similar question LoriLexa replied; "You need to choose ADD If you choose WEAR, it will replace the other layer. If you choose ADD it will allow you to wear multiple layers without replacing them. The same holds true for objects and tattoo layers. For example: If you want to wear a tattoo on your upper arm and another tattoo layer for your eyeshadow, you must wear the first and ADD the rest or just ADD all of them. Same with rings, if you have 3 different rings that you want to wear to your right hand, you just choose ADD for all of them. Same for hats over hair. If both objects are defaulted to wear on your head, you ADD them both so one does not remove/replace the other. etc...
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So it seems that by "ADDing" mesh, prim, system and alphas layers you can create your own unique Second Life ensembles and costumes. So much for putting on your pants one leg at a time.