JPEG
Zigzag Scan
The quantized DCT coefficient is put side by side along horizontal and vertical frequency elements; this called Zigzag Scanning.Generally, the larger data values are scanned first (starting in the upper left hand corner, the low frequency zone), with smaller data values being gathered as the scan moves down and to the right towards the high frequency zone.
The result of the Zigzag Scanning is that the majority of "0" data values are placed side by side.