A #10 envelope is a standard business envelope that is commonly used for mailing letters and documents. It is also known as a "letter envelope" because an 8.5 x 11 inch piece of paper fits in it securely. The dimensions of a #10 envelope are 4.125 x 9.5 inches. The envelope is open on the longer side, and it is often used for mailing bills, invoices, and other business correspondence. It is important to note that while there is no law dictating that #10 envelopes be this precise size, most envelope manufacturers use these standard dimensions as a general rule of thumb.