Much trouble and many after-disputes can be avoided in large operations by requiring in the bid a table of prices, which can be agreed upon beforehand, to be paid for extra work, or to be deducted for work or fixtures which are omitted. In making up the schedule of prices, the plumber can charge his regular rate of wages for his workmen when sent out on jobbing work, and can deduct the time that would be required to do certain work at the regular rate of wages price. The object is not to deprive the contractor of his just profits, but to furnish a basis for the ordering of extra work, for it is extra work instead of omissions which is more often ordered. The cost of fixtures can likewise be listed with a fair and reasonable margin of profit.