Knee-length socks, also known as over-the-calf, should always be worn with a suit and tie, or for that matter, any business occasion. This includes business casual. The reason is that a bare leg should never be visible when sitting. Just as a tee-shirt prevents exposing skin or hair, the knee-length sock covers the leg appropriately.
Other items of interest concerning socks: Use colors that reflect the base color of the pants. For example, with black pants wear black socks and with navy slacks, wear navy socks. A word of caution when working with the color navy: A true navy is very dark, therefore it is sometimes difficult to find a true dark navy sock, so consider black instead.
If you are unsure on the base color of a suit, look at the lining of the jacket. Frequently this will tell you if it is gray, brown, black or navy. Another word of caution, a lace-up, wingtip, plain-toe, or cap-toe shoe should be work with sold socks only, no pattern. You can use a very subtle pattern with your slip-on shoes, provided it is not too loud!
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