Let me also explain machining, which is always a bit tricky to understand. I will be using the names by Motedis. When you order your profiles, you can order them without machining (for instance, when you will be using only brackets), with the M12 machining (not very common, consists in a threaded hole on the inside of the profile, D17 and D17V). The diference between D17 and D17V only affects wide profiles, such as for instance 45x90. For symmetrical profiles such as 45x45, D17 and D17V are the same because you can always rotate the profile when installing it. In the case of wide profiles, D17V is normally the kind of machining you want, because it means the hole will be in the 45 sides. Remember that you will have to specify if you want machining in one end or in both ends. For instance, for the base frame, you will use machining on both ends for the front and rear profiles. For the columns, you only need machining on one end (where they attach to the base). Now, when ordering the quick connectors, T-connectors or whatever they are called, you have to pay attention to how the profiles will connect. You can find 2 types of connectors: 0° and 90°. If you are in dobut, you can also order some bags of each type, hehe. Finally remember that machining is only for the case in which the end of the profiles touches other profile. It doesn't work for this kind of installation (see photo), where you will need brackets.