Definition from Wiktionary, the free dictionary
Cardinal and ordinal numerals 
- The numbers one (uno) and one hundred (ciento) apocopate (have their final sound removed) when they are in front of a masculine noun, an adjective, or another numeral.
- The full form is used for feminine nouns, unless the word begins with a stressed a-.
- The use of a comma or dot to separate thousands varies throughout Latin America and Spain. The Real Academia Española accepts both uses.
- In text, the numbers from 0 to 9 are usually written with words; the numbers above 20 are usually written as numerals.
- From 11 onward, the ordinal numerals can be replaced with cardinal numerals for the same meaning.
- decimoséptimo día, diecisiete día
- The adjectives primero and tercero become primer and tercer when before nouns, or before another adjective that modifies a noun.
- el tercer edificio que he construido
- el primer buen conejo
- If the adjectives are separated by y there is no shortening.
- un tercero y postrero golpe