Suppose a broker receives the following orders to offer to sell stock for company ABC:
200 shares for $1,000
300 shares for $1,500
100 shares for $1800
350 shares for $1,600
At the same time, the following are available bid prices for the same company's stock:
100 shares for $900
200 shares for $800
150 shares for $950
The NBBO for ABC is $950/$1,000 because they are the best bid/offer prices available to traders within the given range.