TAdvPageControl and TAdvTabSet are designed to allow easy and flexible use of tab background bitmaps.  A single bitmap is sufficient to be used as normal tab background and another bitmap as selected tab background. The bitmap's bottom-most line is not used. The color of this bottom most line is used as transparency color. The tab bitmap is split in 3 parts to automatically stretch in an intelligent way to the size of the tab.The leftmost and rightmost half of the background bitmap are used as leftmost and rightmost part in the actual tab background. The middle 2 pixels are stretched to fill the full tab. In this way, a single bitmap is sufficient to draw all tab widths and all corner detail is maximally preserved.  The same applies for bitmaps used for vertical tabs but the mechanism to split the bitmap is rotated 90 degrees.