We can all thank Georges Seurat for his pioneering work in pointillism to advance the theory of, and reproduction of, color. The confusion is in the terminology and different printing technologies used to reproduce the image. Many people incorrectly interchange dpi for Lpi. PPI is for pixels per inch that your monitors resolution is manufactured at. I believe most to be around 92 ppi for crts. DPI (or dots per inch) is for the original image resolution. The actual printed size and method of reproduction should determine the number of dots per inch to render at. SPI (spots per inch) is the terminology for how laser printers & most photo printers output resolutions are measured. Photo printers are typically 1200 + spots per inch horizontally. You can imagine a grid that is 1" wide with 1200 potential spots or color. When the various colors are called for to "print" the nozzles either print that color dot or they don't. It operates much like the old dot matrix printers did, if you remember them, only at a much finer resolution. LPI (lines per inch or screen frequency) is the measure for offset printing screens used to burn the image printing plates, one for each color (CMYK). The LPI will vary depending on the type of paper being used but in general a line screen of 150 or 175 is common for coated offset paper. The general rule of thumb is to use an image resolution of 2 1/2 time the lines per inch. Hence, a one inch final printed image size printed at 150 lines per inch should be 150 LPI x 2 1/2 = 300 dpi. The reason that the image for CMYK offset printing needs to be higher then the one for the ink jet printer is a matter of ink spot size relative to the "spot" at which it is to print. Not only does each color have the potential to be "on or off" for the spot at which it is to print (like for the ink jet printer) but also the "size" of each ink spot is variable between 0 & 100% for that particular spot on the matrix. Short story is to use the 300 ppi (to the actual printed size) as others have said.