Due to the large amount of parallel stories and overlapping material that is shared between Matthew, Mark and Luke, it is generally accepted that Matthew and Luke borrowed or copied material from Mark's Gospel, in addition to other sources, such as the yet unfound Q document (German: Quelle, meaning "source").
There are also theories that additional documents must have existed to account for the unique material that is found only in the Gospels of Matthew and Luke, referred to simply as the M and L sources, which, along with other sources, may have served as pre or proto documents to the Gospels. Many of these source documents would likely have been made from oral transcriptions of the sayings and events.
There are many theories about the original source documents that contributed to the "modern" Gospels of Matthew and Luke, and these origin hypotheses are noted in simplest terms by the number of source documents involved.
While it is interesting and useful to know which of the four accounts was written first, and which author borrowed or copied what material from whom, and to speculate about what their original source documents may have been some two thousand years ago, the purpose of The Synoptic Gospel is to align and harmonize the words of the texts of the four Gospels of the New Testament as they exist, in order to produce a single, complete Gospel account of the life and teachings of Jesus Christ.
