Waiver.  If  a  party  to  a  contract  clearly  waives  his  right
under  a  certain  clause,  and  the  other  party,  relying  on  such  a
waiver,  does  not  fulfil  that  clause,  the  resulting  variation  from  the
contract  terms  is  excused.  Thus  if  a  building  contract  makes  an
architect's  certificate  a  condition  precedent  to  payment,  and  if
the  owner  gives  the  contractor  to  understand  that  the  certificate
will  not  be  required,  this  waiver  renders  the  production  of  the  cer-
tificate unnecessary  as  a  condition  of  payment.  Waiver  may  be
fouiKl  ^n  the  conduct,  as  well  as  in  the  words,  of  a  party.  The
■^'lestion  whether  words  or  conduct  amount  to  a  waiver,  is  one  for
the  jnry  to  decide.
Modification.  Contracts  may  always  be  modified  by  agree-
ment of  the  parties.  The  subsequent  agreement  must  be  a  good
contract  according  to  the  ordinary  rules,  in  order  to  effect  a  change


329


16  LEGAL  RELATIONS

in  an  existing  contract.  A  waiver  may  also  practically  modify  a
contract.  It  is  important,  however,  for  the  sake  of  avoiding  com-
plications  and  disputes  to  have  a  contract  always  kept  clear  and
readily  provable.  Hence  it  is  only  a  reasonable  precaution  to  have
any  modifications  of  a  written  contract  put  in  writing.  One  advan-
tage of  this  is  that  the  written  expression  of  the  understanding  is
likely  to  be  made  more  definite,  thus  avoiding  disputes  on  the  ground
of  inexactness,  as  well  as  on  the  ground  of  misunderstanding.
Suretyship.  The  relation  of  suretyship  arises  where  besides
an  obligee,  as,  for  instance  a  creditor,  and  the  principal  obligor,  as
a  debtor,  there  is  another  person  who  becomes  answerable  for  the
debt  or  default  of  the  principal  obligor,  and  who  is  called  a  surety.
