IT architecture is the discipline of linking business to IT. Starting with your company's vision and mission via your strategy how to get there and your processes to support this down to the applications and finally projects that implement the required process support IT architecture maintains the link between the levels. To achieve this many decisions have to be taken. On the top levels assessments help you decide on what to focus on, and on the lower levels evaluations guide you on how to do things or what products to choose. While assessments and evaluations as best practice IT architecture methodologies ensure objective and reproducible decisions they will still not result in a coherent IT architecture without guiding IT architecture principles that ensure that assessment and evaluation criteria as well as the measures of relative importance and fulfilment levels are coherent throughout all engagements.
There might be different ways to develop your IT architecture principles but the way I would recommend is to derive them directly from your business IT strategy. Based on your companies vision and mission such a document lines out your business imperatives and supporting IT strategy. Since your IT architecture principles will be the guard rail for all of your IT architectural decisions to ensure that business and IT will be aligned your business IT strategy is the information to start from. If you needed some IT architecture principles' examples a good starting point would be The Open Group Architecture Framework (TOGAF).
