The following practices may be said to characterize monopolies.
Exclusive dealing to supply and collective boycott
Producers agree to supply only to recognized dealers, normally only one dealer in each area, on condition that the dealer does not stock the products of any producer outside the group (or trade association). Should the dealer break the agreement, all members of the group agree to withhold supplies from the offender. This practice has proved a very effective restriction on competition for it ensures that any new firms would find it extremely difficult to secure market outlets for their products.
The creation of barriers to ensure that there is no competition against them. E.g price undercutting, individual ensure that actual text printed collective boycott and exclusive holding of patent rights.
Resale Price Maintenance (PRM)
A monopolistic firm may dictate to wholesalers and retailers the price at which its products would be sold. This is another way of ensuring that other firms are not attracted into the industry, if such firms can sell their products at more competitive prices.