diff --git a/pygmt/alias.py b/pygmt/alias.py
new file mode 100644
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--- /dev/null
+++ b/pygmt/alias.py
@@ -0,0 +1,134 @@
+"""
+The PyGMT alias system to convert PyGMT long-form arguments to GMT's short-form.
+"""
+
+from collections.abc import Mapping, Sequence
+from typing import Any, Literal
+
+from pygmt.exceptions import GMTValueError
+from pygmt.helpers.utils import is_nonstr_iter, sequence_join
+
+
+def _to_string(
+    value: Any,
+    prefix: str = "",  # Default to an empty string to simplify the code logic.
+    mapping: bool | Mapping = False,
+    separator: Literal["/", ","] | None = None,
+    size: int | Sequence[int] | None = None,
+    ndim: int = 1,
+    name: str | None = None,
+) -> str | list[str] | None:
+    """
+    Convert any value to a string, a sequence of strings or None.
+
+    The general rules are:
+
+    - ``None``/``False`` will be converted to ``None``.
+    - ``True`` will be converted to an empty string.
+    - A sequence will be joined by the separator if a separator is provided. Otherwise,
+      each item in the sequence will be converted to a string and a sequence of strings
+      will be returned.
+    - Any other type of values will be converted to a string if possible.
+
+    If a mapping dictionary is provided, the value will be converted to the short-form
+    string that GMT accepts (e.g., mapping PyGMT long-form argument ``"high"`` to GMT's
+    short-form argument ``"h"``). If the value is not in the mapping dictionary, the
+    original value will be returned. If ``mapping`` is set to ``True``, the first letter
+    of the long-form argument will be used as the short-form argument.
+
+    An optional prefix (e.g., `"+o"`) can be added to the beginning of the converted
+    string.
+
+    To avoid extra overhead, this function does not validate parameter combinations. For
+    example, if ``value`` is a sequence but ``separator`` is not specified, the function
+    will return a sequence of strings. In this case, ``prefix`` has no effect, but the
+    function does not check for such inconsistencies. The maintaner should ensure that
+    the parameter combinations are valid.
+
+    Parameters
+    ----------
+    value
+        The value to convert.
+    prefix
+        The string to add as a prefix to the returned value.
+    mapping
+        A mapping dictionary or ``True`` to map long-form arguments to GMT's short-form
+        arguments. If ``True``, will use the first letter of the long-form arguments.
+    separator
+        The separator to use if the value is a sequence.
+
+    Returns
+    -------
+    ret
+        The converted value.
+
+    Examples
+    --------
+    >>> _to_string("text")
+    'text'
+    >>> _to_string(12)
+    '12'
+    >>> _to_string(True)
+    ''
+    >>> _to_string(False)
+    >>> _to_string(None)
+
+    >>> _to_string("text", prefix="+a")
+    '+atext'
+    >>> _to_string(12, prefix="+a")
+    '+a12'
+    >>> _to_string(True, prefix="+a")
+    '+a'
+    >>> _to_string(False, prefix="+a")
+    >>> _to_string(None, prefix="+a")
+
+    >>> _to_string("high", mapping=True)
+    'h'
+    >>> _to_string("mean", mapping={"mean": "a", "mad": "d", "full": "g"})
+    'a'
+    >>> _to_string("invalid", mapping={"mean": "a", "mad": "d", "full": "g"})
+    Traceback (most recent call last):
+    ...
+    pygmt...GMTInvalidInput: Invalid value: 'invalid'. Valid values are: mean, ...
+
+    >>> _to_string((12, 34), separator="/")
+    '12/34'
+    >>> _to_string(("12p", "34p"), separator=",")
+    '12p,34p'
+    >>> _to_string(("12p", "34p"), prefix="+o", separator="/")
+    '+o12p/34p'
+
+    >>> _to_string(["xaf", "yaf", "WSen"])
+    ['xaf', 'yaf', 'WSen']
+    """
+    # None and False are converted to None.
+    if value is None or value is False:
+        return None
+    # True is converted to an empty string with the optional prefix.
+    if value is True:
+        return f"{prefix}"
+    # Any non-sequence value is converted to a string.
+    if not is_nonstr_iter(value):
+        match mapping:
+            case False:
+                pass
+            case True:
+                value = value[0]
+            case Mapping():
+                if value not in mapping and value not in mapping.values():
+                    raise GMTValueError(
+                        value,
+                        description="value for parameter {name!r}" if name else "value",
+                        choices=mapping.keys(),
+                    )
+                value = mapping.get(value, value)
+        return f"{prefix}{value}"
+
+    # Return the sequence if separator is not specified for options like '-B'.
+    # True in a sequence will be converted to an empty string.
+    if separator is None:
+        return [str(item) if item is not True else "" for item in value]
+    # Join the sequence of values with the separator.
+    # "prefix" and "mapping" are ignored. We can enable them when needed.
+    _value = sequence_join(value, separator=separator, size=size, ndim=ndim, name=name)
+    return f"{prefix}{_value}"