VBlank is the period when the NES [and video screen] has finished drawing an entire screen [or frame] of the image, and the electron beam of the video screen is moving back up from the bottom-right of the screen to the top-left, to begin drawing the screen anew. During this crucial period, it is safe for your NES programs to write to the PPU without fear of glitching. The VBlank period is not long enough to make extensive writes to the PPU, but it is still long enough to make some small changes [such as to the palette or a few CHR tiles in CHR-RAM]. VBlank is also an essential way to get constant timing in game physics and music. On NTSC NESes, there are 60 VBlanks per second; there are 50 per second on PAL NES machines. HBlank is the period on each scanline when the electron beam moves back from the right side of one scanline to the left side of the next scanline. Generally, changes to scrolling and name table updating will not become apparent until after the next HBlank. So, say for