  Using E-strings, we can analyze not only six-dimensional superconformal field
theories but also probe vacua of non-perturabative heterotic string. We study
strings made of D3-branes wrapped on various two-cycles in the global F-theory
setup. We claim that E-strings are elementary in the sense that various
combinations of E-strings can form M-strings as well as heterotic strings and
new kind of strings, called G-strings. Using them, we show that emissions and
combinations of heterotic small instantons generate most of known
six-dimensional superconformal theories, their affinizations and little string
theories. Taking account of global structure of compact internal geometry, we
also show that special combinations of E-strings play an important role in
constructing six-dimensional theories of $D$- and $E$-types. We check global
consistency conditions from anomaly cancellation conditions, both from
five-branes and strings, and show that they are given in terms of elementary
E-string combinations.
