In Histogram of Oriented Gradients, the edge orientations within rectangular patches are binned into the bins of a histogram. Each pixel adds the strength of its orientation in the bin corresponding to that orientation. The image pixels are grouped in a regular grid of these patches, which are called 'cells'.
In the next layer, overlapping patches of cells are combined in order to create blocks. In these blocks the histograms are normalized.
1) How are the cells combined into a block? Are the cell histogram bin values added together, or are the bins of the cell histograms concatenated, forming one large histogram?
Suppose the cell histograms have $9$ bins and the blocks contain $2\times 2$ cells. Do the block histograms have $9$ bins, or $9\times2\times2 =36$ bins?
2) Why is adding or concatenating better than the other?