In these systems, one person volunteers to work for an hour for another person; thus, they are credited with one hour, which they can redeem for an hour of service from another volunteer. Others use time units that might be fractions of an hour (e.g. minutes, ten minutes – 6 units/hour, or 15 minutes – 4 units/hour). While most time-based exchange systems are service exchanges in that most exchange involves the provision of services that can be measured in a time unit, it is also possible to exchange goods by 'pricing' them in terms of the average national hourly wage rate (e.g. if the average hourly rate is $20/hour, then a commodity valued at $20 in the national currency would be equivalent to 1 hour).