1000 - #9 SELF SEAL Double Window Check Envelope - 3-7/8" X 8-7/8" - For Quickbooks, Quicken and other Accounting programs

While not the least expensive option, I tried these as an alternative to the supplier I used to use. The envelopes are slightly larger so the checks can slide around, but the flaps being already open makes it easier to stuff a volume of check at a time with less impediment. If the envelopes were a better size for checks I would probably switch over completely.