The letter m does more than one job in SI notation. By itself after a number it can be the metre; attached to another unit it can be the prefix milli. Change it to uppercase M and it becomes the prefix mega.
So the expression has to be read as a structure. Case tells you part of the story; position tells you the rest.
One letter, three roles
| Form | Interpretation | Where it sits | Example |
|---|---|---|---|
| m | metre | SI base-unit symbol | 5 m |
| m | milli = 10⁻³ | SI prefix | mm, ms, mV |
| M | mega = 10⁶ | SI prefix | MHz, MW, MPa |
m as metre
Length in SI is based on the metre, whose symbol is lowercase m. A value such as 2 m shows the ordinary number–unit boundary used throughout measurement notation.
When m appears by itself as the unit after a numerical length, there is no prefix involved. It is simply the metre symbol.
m as milli
When lowercase m is attached to another unit, it can instead be the prefix milli, a factor of 10−3. Thus mm is read structurally as milli + metre, not as two independent metre symbols.
You see the same grammar in ms for millisecond and mV for millivolt. Milli has meaning only as part of that combined unit token; there is no space inserted between prefix and unit.
M as mega
Capital M is the SI prefix mega, representing 106. Examples include MHz for megahertz, MW for megawatt, and MPa for megapascal.
Changing a prefix from lowercase m to uppercase M changes the factor from 10−3 to 106—a difference of nine orders of magnitude. mW and MW are therefore not stylistic variants of the same unit.
mm vs Mm: one changed letter, nine orders of magnitude
mm is millimetre: milli (m) + metre (m). Mm is megametre: mega (M) + metre (m). They differ only in the case of the first letter, yet one is 10−3 metre and the other 106 metres.
This is ordinary SI prefix grammar, not a Unicode trick. Both symbols use the same basic Latin letters; the semantics come from the standardized case-sensitive notation.
Why a space changes the expression
Compare ms with m s. The first is millisecond because milli is attached to the second symbol. With a space, m and s are separate unit symbols and can be read as a product of metre and second in a context where that compound unit makes sense.
That difference is why whitespace should not be stripped blindly from scientific data. Prefix attachment and unit multiplication are represented differently.
Micrometre is a separate prefix story
Micrometre uses the micro prefix followed by the metre symbol. In digital text, more than one mu-shaped Unicode character can appear, and ASCII systems sometimes substitute um.
The dedicated guide to µm/μm vs um covers that encoding problem. The important connection here is that the final lowercase m remains the metre unit; the preceding character is the prefix.
Case conversion can wreck a value
Generic text cleanup may lowercase headings, uppercase labels, or apply title case. Those operations are dangerous inside unit expressions. Converting mW to MW, or mm to Mm, changes the magnitude rather than merely the appearance.
The same case sensitivity appears in k vs K: lowercase k is kilo, while uppercase K is the kelvin unit.
How a parser can tell the roles apart
Do not assign meaning to every isolated m or M in scientific prose. The same letters can be variables, names, identifiers, or ordinary text. First identify that a span is a unit expression.
Inside that expression, parse the complete token. A standalone m after a number can be metre; m attached to another unit can be milli; uppercase M attached to a unit is mega. Preserve case and spacing before interpretation, because both are part of the notation.
Until the unit structure is known, preserve the text exactly. Lowercase m may be metre or milli depending on position; uppercase M is mega. Changing case or spacing before parsing can destroy the evidence needed to tell those roles apart.
Why milli and mega cannot stand alone
Both milli and mega are prefixes, so they need a unit to modify. mV and MV are complete prefixed unit symbols; a bare m or M is not a complete expression for milli or mega by itself.
Compare 5 m with 5 ms. In the first, m is the complete unit symbol for metre. In the second, the first m is a prefix attached to s. The character is the same; its grammatical role inside the unit expression is not.
Variables add one more ambiguity
Equations add another layer: authors also use m and M as quantity symbols or variables. A letter chosen for mass, for example, is not automatically the metre symbol simply because it is m.
Formal typesetting often helps by setting unit symbols upright and quantity symbols in italic, but plain text loses that cue. Software then has to rely on structure: what follows the number, whether the token matches a known prefixed unit, and whether the letter appears as a variable in an equation.