Ejemplo de posicionamiento absoluto.

main

CSS:

            * {
                margin: 0;
                padding: 0;
                box-sizing: border-box;
            }
            .contenedor {
                position: relative;
                width: 1600px;
                height: 800px;
                border: 1px solid;
                margin: 0 auto;
            }
            header {
                background-color: tomato;
                height: 200px;
            }
            aside {
                position: absolute;
                top: 200px;
                left: 0;
                width: 400px;
                height: 600px;
                background-color: bisque;
            }
            main {
                position: absolute;
                top: 200px;
                left: 400px;
                width: 1200px;
                height: 600px;
                background-color: whitesmoke;
            }