The Swedish branch of this common tongue developed into a separate language during the period 900-1500 and is called Old Swedish. Until after 1200 the only records of the language are runic inscriptions, cut primarily on tombstones and memorial stones. The Latin alphabet was introduced in the 13th century; periods of further differentiation followed, and some approximation to Danish occurred. The written language, based on two of the most widely spoken dialects, was made uniform throughout all of Sweden in the 14th century. Aside from differences in vocabulary, Swedish now differs from Danish especially in its retention, after a vowel, of the old voiceless consonants k, t, and p, which in Danish changed to g, d, and b, and in its retention of the vowels a and o in unstressed syllables, whereas Danish has e or no vowel.

The main body of the Swedish vocabulary is of old Germanic stock. The principal foreign ingredients are Latin and Greek words that came in with Christianity and also with the growth of scholarship. Low German words dating from the time of the Hanseatic League (13th-16th century), German words from the 17th century, and French words borrowed in the 17th and 18th centuries are all part of modern Swedish. Swedish has a unique accent that involves both stress and variations in musical pitch, which gives it a singsong rhythm. A simplified spelling reform was introduced in 1906. Even today, great dialectical differences exist in spoken Swedish throughout the country, and some linguists still think some of them are separate languages, e.g. Skanian.