invitation to treat- advertisement
Generally an advertisement cannot be an offer and is usually considered to be an invitation to treat
However, if the advertisement is a unilateral offer or has certain terms that need to be fulfilled it can be considered as an offer.
partridge v crittenden-Facts: Mr Crittenden placed an advertisement in stating "Bramblefinch cocks, bramblefinch hens, 25s". He was prosecuted for 'offering for sale' a wild bird under the Protection of Birds Act 1954.
Ratio: Mr Crittenden had not made a legal offer but instead an invitation to treat.