Simply #10 Security Envelopes with QuickStrip Closure - 4-1/8" x 9-1/2" - White - 500 PackIdeal for home and office use, Simply #10 Security Envelopes with QuickStrip Closure are a versatile choice. #10 is a commonly used envelope size for mailing letters, cheques, billing statements, invoices and more. These envelopes feature a QuickStrip closure that seals without moistening. Simply remove the strip and press the square flap down to seal. A security tint keeps contents private, preventing others from reading confidential information.