Ada Gilmore, printing block. Courtesy of Julie Heller Gallery, Provincetown, MA
An edition is understood to be the total number of prints taken from a printing matrix, usually printed at the same time. Here is a quote taken from the International Fine Print Dealers Association (IFPDA) website regarding the numbering of editioned prints:

"While the numbering of individual impressions can be found as early as the late nineteenth century, it did not become standard practice until the mid-1960s. Before steel-facing and other ways of preserving plates for longer print runs, the order in which the edition was printed was important. An intaglio plate, especially one containing drypoint lines, will degrade over time as the pressure of the press will dull the burr. As a result, the first impression is often crisper than the last and in turn, the edition should be numbered in order. Today, all limited edition prints should be numbered, and because of advancements in technology and a printer’s ability to print reciprocal, identical images, the numbering sequence is no longer intended to reflect the order of printing. Numbering is now transcribed as a fraction with the top number signifying the number of that particular print and the bottom number representing the total number of prints in the edition. The edition number does not include proofs, but only the total number of prints in the numbered edition."

This is all well and good, as long as we're talking about editionable fine art prints such as etchings and lithographs. However, white line woodcuts are really quite a different animal. Though technically they are "printed" from a block, each print, having been painstakingly painted shape by shape and color by color, is as unique as a fingerprint. For many years, I worked as a professional printer for other artists of reputation, and my goal was to match a "bon a tirer," (or proof that the artist approved for me to match) to no more than about a 3% variation from the original artist's proof. The goal of white line printing is about as opposite to that as possible! It's all about freedom of expression. The white line prints are more like paintings or monoprints; variation in color and density is cause for celebration, not condemnation. 

Considering the unique nature of the genre, I've seen a few different methods of numbering the white line prints. As for myself, I began simply to number each print sequentially (No. 1, No. 2, etc.) that I printed from a particular block. Only recently, a print dealer asked me to state a limit to the number of prints that I'd take from a block, and suggested 25 as a reasonable number. This is driven by the consumer, who wants to know that his investment will not be "diluted" by the possibility of an unlimited number of impressions in circulation. When you think about the amount of labor required to produce one print, however, 25 is terribly optimistic! I've begun to number my prints 1/25, 2/25, etc., but I may not live to complete that number, especially of some of my larger pieces. I'm keeping good records, but I have many blocks, and really am more interested in creating new images than in slavishly reproducing a successful "copy" of an image. Even Blanche Lazzell only produced 4 or 5 prints of each of her blocks. Of course, she didn't have the internet to help market her work, but the point is that she was more interested in variation of color in her individual prints. At any rate, she came up with her own numbering system:

 "When the artist began her record book, she devised a unique numbering scheme for keeping account of her prints. Lazzell listed these numerals in her catalogue and wrote them on the woodcuts, often in each of the four corners of the image on the verso of the sheet. The inscriptions have long baffled collectors, since they look like transposed edition numbers. Lazzell’s bipartite catalogue numbers resemble fractions. Their first number—in the numerator position—represents the woodcut in the sequence of all of Lazzell’s white-line prints, functioning something like a musicologist’s opus number. The digits behind the slash—in the denominator position—number the impressions from a particular block. Soon the artist began to inscribe each print in detail with its title, her catalogue number, when the block was cut, and where and when the impression was printed."

So, you might see a Blanche Lazzell numbered (on the back) thus: 154/3, which meant that it was her 154th block, and the third print taken from that block. It worked for her. Interesting to note that she often signed her prints within the image, as painters do. “The wood block print has the same rank in art as any other medium or form of expression," she wrote. And I can only agree.