Quality Park #9 Double-Window Envelopes, Left Windows (Top/Bottom), Gummed Seal, White, Box Of 500 - Quality 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. These 24 lb white wove envelopes have side seams for a clean, professional look.