Word for a letter of the alphabet that helps form another letter

Solution 1:

Some relevant terms include typographic ligature, which refers to a symbol or character made from multiple letters:

Two or more letters combined into one character make a ligature. In typography some ligatures represent specific sounds or words such as the AE or æ diphthong ligature. Other ligatures are primarily to make type more attractive on the page such as the fl and fi ligatures. In most cases, a ligature is only available in extended characters sets or special expert sets of fonts.

Also relevant would be grapheme:

the smallest meaningful contrastive unit in a writing system.

And then there are glyphs:

The shape given in a particular typeface to a specific grapheme or symbol. Most commonly glyphs are letters and numerals, but punctuation marks and symbols and shapes (e.g. ITC Zapf Dingbats) are also glyphs.

So, for example “fi” comprises two glyphs, namely “f” and “i”; however, they can be combined into a single “fi” glyph (in this instance the combination of “f” + “i” to form a single glyph, is know as a ligature).

Roughly speaking, one glyph usually corresponds to one character in a typeface; however, there are numerous instances when this is not strictly true. For example, the glyph “ö” is sometimes represented by two characters, one representing the “o”, the other representing the diacritical mark “¨”.

It seems like none of these is an exact answer to your question, as with your example of "P" and "R." But they do provide terminology for how characters can interact with each other to present a larger whole (as with ligature).

I'm guessing typographers would describe letters in relation to each other based on their component parts. For instance, they might say that a capital "R" is like a capital "P" with a leg stroke.