אחד ואחר

The Vilna Gaon (brought on the pasuk לך רד כי שחת עמך) points out beautifully that the one and only ד רבתי is שמע ישראל ה אלהינו ה אחד while the one and only ר  is לא תשתחוה לאל אחר.
[ ובספר מתוק מדבש פר' חקת מכונים מלכות דאצילות ומלכות דקליפת נוגה].

I'd like to point out that visually, the difference between a ד and a ר is that only the ד points, protrudes, and leans towards the preceding letters before it  (casting itself in the context of its primal roots from which it emerged), while the ר  turns its back, presenting itself as a fully independent extant item, divorced from it's past.

So for example, one man can order a steak in a restaurant and purely relate to the present external shell that is before him, a nice tasty piece of meat. Whereas another, in his conscious mind, is eating it while being mindful of the full gamut of history and existential context within which this piece of meat is currently being chewed in his mouth (the gargantuan task of raising the cows, slaughter, processing, etc, the evolution of cows over the millenia, all the way back to the existential underpinnings of כל רמש אשר הוא חי לכם יהיה לאכלה)  So that what comes out is that the difference between a ד and a ר (and consequently between a ה' אחד and an אל אחר) is purely one of consciousness/mindfulness of context, source, existential roots. It's a subtle yet fundamental difference in consciousness, not in physical action.

It's that mindfulness/awareness/consciousness (זכירה) of present Life manifestations in the context of their primal, fundamental, existential origins that provides the perfect male partner (זכר) to the feminine emergent, manifested reality of מלכות. Without that "looking back from whence this comes" leaves you with the ר of an אל אחר.