CHART: Plastic players American Express hopes to gain a greater share of the credit card market with a new and improved Optima card to compete with Visa and Mastercard. Total share fo card market: Visa 38%, MasterCard 24%, American Express 16%, Other:(Discover, Carte Blanche, Diners Club) 22% Visa and MasterCard are accepted in nearly 12 million locations. American Express is accepted in 4 million locations. Grace Period: American Express: Interest charges start accruing on new purchases 25 days after end of billing period, whether or not you carry a balance. Example: If you charge something Oct. 3 and the billing period ends Oct. 20, interest charges on new purchases start Nov. 14. Most others: Interest charges on new charges start accruing the day purchases are made if cardholders carry a balance. Example: If you charge something Oct. 3, the interest begins accruing that day. ASSOCIATED PRESS