Some books will allow you to do an open parlay (see parlay definition here) and allow you to pick your plays over time. These plays are rarely offered. An open parlay will begin with your initial plays and stay open until one of your plays loses. When an open parlay bet is places, the payout on the parlay is usually based on the open spots being a -110 line. As your plays are added your payout will adjust based on the new odds.