The contract of employment is the agreement made between the employer and the employee. T his could be a written agreement or in an oral form. I n addition, the contract will also include “ custom and practice ” agreements. T hese are how things are usually done in the workplace, for example, if the employer always gives the employees a day ’ s holiday in August. E ven though this is not mentioned in the written contract, this will form part of the contract as it is the usual practice. A trade union may have negotiated ( 谈判达成 ) an agreement with an employer about conditions at work. T he negotiated agreement will often form part of a contract. S ome contracts will be illegal if: ※ the employee gets all or part of their wages in cash; ※ tax and national insurance contributions ( 国民保险税 ) are not paid; and ※ the employee knows they are getting paid in this way to avoid paying national insurance and tax.