ZIP has become somewhat of an industry standard, and is supported by\neverything and everybody. It has been overtaken by other formats in\nterms of performance and features, but it still remains the most widely\nused and supported archive format.\n\nOne of the benefits of alternative formats is not only that compression\nratios alone are higher. But a lot of them also supports something\ncalled solid archives. That is, redundancy is removed not only within\neach single file, but also between all files in the archive. In some\ncases, the differences in compression ratios caused by solid archiving\ncan be extreme.\n\nMy question is: Can solid archives somehow be emulated without breaking\ncompatibility with the ZIP standard?\n\nPoint in case: I have a bunch of similar files. Let's look at only a\ncouple of them, say file1 and file2. They are both 464 kB in size.\nCompressed to individual ZIP files they both shrink to 403 kB each.\nZIPping both into one single ZIP gives me a larger ZIP file of 806 kB,\nwhich is to be expected.\n\nNow, let's try something else. I compress file1 and file2 individually\ninto 7-Zip files. The resulting files are 399 kB, i.e. slighly smaller\nthan the ZIP files. However, compressing file1 and file2 into one\nsingle 7z file takes only 400 kB! Adding even more of these files (I\nhave a bunch of them) only seems to increase the 7z archive by about 1\nkB each.\n\nAll examples are made using TugZip, maximum compression, unless\notherwise stated.\n\nNext, I tried to first put file1 and file2 into a container without\ncompressing them (i.e. ZIP with no compression), resulting in a single\nuncompressed 929 kB file, and then compressing this. I was disappointed\nto find that compressing this container with ZIP gave me a 805 kB file,\nonly slightly smaller than the standard ZIP. Compressing the container\nusing 7-Zip yet again produced a 400 kB file. Why this difference? Does\nit have something to do with search span or dictionary size of the two\nalgorithms? Can this difference somehow be worked around?\n\nOut of curiousity I also tried making tgz (.tar.gz) and tbz (.tar.bz2)\narchives of file1 and file2, since these formats are also solid. The\nresulting archives were 805 kB and 505 kB respectively.\n\nThe reason for my concern with this is that I routinely receive and\nsend lots of files to various recipients in my work, either via e-mail\nor from closed web download sites. In particular, mails bouncing due to\nattachment sizes are a common problem. I have tried convincing some of\nmy contacts to consider the possibility of using something like .7z, so\nfar without results. From what I can gather, people are either using\nWinZip or the builtin shell extension in Windows XP. Self extracting\nexecutables are also out of the question, since these are commonly\nblocked due to security policies of various companies.