Let us say that you have a patient with pronation and supination tendencies. They pronate mainly at contact, but due to chronic ankle instability, love to misstep and supinate at times (typically also at contact). For the pronation, you estimate they need a 30 degree inverted correction. For the supination tendency, they have a forefoot valgus/plantar flexed first ray we can use. So, you order a Fettig. You can also typically add a high lateral heel cup and lateral phalange, a Denton modification, and a full topcover to a forefoot valgus extension under the 4th and 5th metatarsal heads.
Let us say that you suspect initially, or you see at dispense, that the patient is not as controlled laterally as you would like. Your next orthotic device will be a lateral Kirby skive to the above cast, a Feehery cuboid skive, or both. And you can also add more height to the lateral heel cup, and more height to the lateral phalange, along with a bigger forefoot valgus forefoot extension. The possibilities are endless. Please ask other questions. Rich
PS I use the Denton routinely (almost daily) and the Fettig modification 1-2 times per year. I probably use the Feehery once every other month. I relie a lot on stable shoes, lateral phalanges, forefoot valgus extensions.