An invitation saying "black tie" used to mean formal evening dress, where rules were rigidly observed: men wore black dinner jackets and trousers, white dress shirts and black hand-tied bow ties; women wore long or short dresses in rich fabrics or the classic "little black dress."
Today's interpretation of black-tie formality is more relaxed, and personal style is acceptable: for men, colorful bow ties in rich fabrics, but still hand tied, are perfectly correct with or without a matching cummerbund—and women can wear nice pant suits.
The conventional dinner jacket can quite easily be exchanged for a brocade or velvet one. For formal summer occasions, light-colored linen suits as well as a white dinner jacket and black evening trousers are all acceptable dress.
Renting an evening outfit makes financial sense especially if yours is an once-in-a-lifetime function, if your weight fluctuates, or if you want to wear the most fashionable styles. Women's formal-wear rental agencies often have complete designer outfits for a fraction of the real cost. However, for men who have many black-tie engagements, buying a classic dinner jacket and trousers is a practical idea.
If you are looking to purchase eveningwear, rental shops may sell some of their stock. It is often also worth the effort to shop at second-hand and charity outlets or garage sales. Many outfits have hardly been worn and go for bargain prices. Look out for broken zippers, damaged linings, and heavy perspiration stains (which are rarely successfully cleaned). Moth holes and cigarette burns are also very expensive to repair.
For a good fit it is worth enquiring at your dry cleaners about alterations: sleeves can be shortened and hems expertly let down and taken up. Large sizes can be altered to several sizes smaller, but a small size can rarely be altered to a larger one.