There are three primary methods to control the size of the front triangle. Each method allows control of two distinct dimensions. Notice that if you choose to control the front center distance, then you will also have control of the head tube length. If you choose to control effective top tube length, then you will also have control of the top tube angle. If you choose to control the horizontal position of the handlebar relative to the bottom bracket, then you will also have control of the vertical position of the handlebar relative to the bottom bracket.
Remember that when you select a pair of dimensions from this list, not only are these the dimensions that you can use to drive the geometry, they are also the dimensions that will remain fixed as you change other dimensions on the bike. For example, if you choose Handlebar from BB (X and Y) to set the handlebar position, you can later go and change the stem length and the handlebar will remain fixed. The frame will adjust to accommodate the change in the stem.
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