Double Window Security-Tinted Check Envelope, #9, Commercial Flap, Gummed Closure, 3.88 x 8.88, White, 500/BoxQuality Park Double Window Envelopes display both sender and receiver addresses for mailing statements and invoices. These security envelopes have a reflex blue pattern that protects sensitive client information because it obscures the details of the contents, even when held up to light.