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The following letters all have something in common which may not be obvious at a first glance:

A B D H P

No other letters share this attribute.

Hint

There are no misspellings or typos in the title of this question. Maybe a clue though.

More hints may follow if the question is not answered.

Some great answers so far all of which I have upvoted but none of which are exactly what I am looking for.

Hint 2

The number 0 and the character ( also have the same property. I only said no other letters share it ;-)

Hint 3

As correctly identified by @RedBaron the ASCII table is key here. There is a good reason why "sum" is mentioned in the title and there are two reasons why "two" is mentioned.

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    $\begingroup$ Are you sure B shouldn't be included too? $\endgroup$
    – Deusovi ♦
    Commented Apr 18, 2019 at 9:18
  • $\begingroup$ @Deusovi You are indeed correct. I had missed that. I'll update the question. Thanks. $\endgroup$
    – ElPedro
    Commented Apr 18, 2019 at 9:19
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    $\begingroup$ Does a ! also share this property? $\endgroup$
    – Eagle
    Commented Apr 18, 2019 at 12:04
  • $\begingroup$ @Akari Yes it does. I have not listed them all. $\endgroup$
    – ElPedro
    Commented Apr 18, 2019 at 12:06

5 Answers 5

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The property seems to be related to:

Binary equivalents of the symbols/alphabets etc.

Explanation:

Binary equivalents for the following can be written as:
A -- 01000001
B -- 01000010
D -- 01000100
H -- 01001000
P -- 01010000
0 -- 00110000
( -- 00101000

So the property is,

The sum of digits in the binary equivalents is two

Or

The binary equivalents of all these have two 1s and six 0s.
Looking at the binary equivalents of the alphabets, one can see that no other alphabets share this property

The title (Thanks to @trolley813):

Two might refer to the sum of the digits, which is indeed two!
Title might mean that the sum [of digits in] letters is not different from two [but is equal to two]

Old (and wrong) answer

The property is:

The index if each alphabet is equal to the sum of indices of the preceding alphabets in the sequence +1.

And,

There is no other alphabet with the index 1+2+4+8+16+1 = 32

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    $\begingroup$ You've got it. Binary was what I was looking for but there are some other interesting patterns came out of this puzzle. I have added another hint in case anyone wants to have a go without looking at your answer. For the same reason, I'll wait a couple of hours before I accept it. well done! $\endgroup$
    – ElPedro
    Commented Apr 18, 2019 at 12:26
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    $\begingroup$ @Eagle Some refinement about the title, it probably should read "sum [of digits in] letters is not different from two [i.e. equals to 2]" $\endgroup$
    – trolley813
    Commented Apr 19, 2019 at 10:03
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    $\begingroup$ Thanks a lot @trolley813 ! I've edited it $\endgroup$
    – Eagle
    Commented Apr 19, 2019 at 11:15
  • $\begingroup$ @trolley813 - Close but actually more of a play on words. Rot13(Fhz (Fbzr) yrggref ner abg gjb (gbb) qvssrerag) with Rot13(Fhz) and Rot13(gjb) giving clues to what I was looking for ;-) $\endgroup$
    – ElPedro
    Commented Apr 19, 2019 at 14:51
  • $\begingroup$ A bit contrived, I know, but left room for a couple of hints. $\endgroup$
    – ElPedro
    Commented Apr 19, 2019 at 14:58
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The property is that

each of their alphanumeric values (A=1, B=2, C=3...) is a power of 2.

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    $\begingroup$ That wasn't what I was looking for but is indeed true and is possibly a side effect of the answer that I was looking for so +1 but I won't mark it as accepted yet. $\endgroup$
    – ElPedro
    Commented Apr 18, 2019 at 9:23
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    $\begingroup$ @ElPedro: It is a side effect, but it hinges on rot13(jurer gur nycunorg fgnegf ba gur NFPVV gnoyr. Vtaber gur svefg ovg bs gur NFPVV inyhr (orpnhfr vg vf nyjnlf 1 naq arire punatrf sebz N gb M), ohg QB erzrzore gung vg nyjnlf pbagevohgrf gb bar bs gur gjb 1-ovgf va lbhe vagraqrq nafjre. Vtabevat gur svefg ovg, N rssrpgviryl fgnegf ng ahzrevpny inyhr 1, naq rirel "cbjre bs gjb" yrggre nqqf rknpgyl 1 1-ovg gb gur pbhag, juvpu rkcynvaf jul obgu nafjref ner pbeerpg. Vs N unq fgnegrq ba n qvssrerag NFPVV inyhr, vg znl abg unir orra gur pnfr.) $\endgroup$
    – Flater
    Commented Apr 19, 2019 at 8:21
  • $\begingroup$ @Flater - Thanks for the explanation. It makes complete sense. As I said, some pretty interesting things have come out of what I at first though was a pretty simple puzzle :) $\endgroup$
    – ElPedro
    Commented Apr 19, 2019 at 14:43
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Is it

All the letters, symbols in this group have ASCII codes of form $2^m + 2^n$ where m and n are integers

Thus we have

From ascii code table,
$A = 65 = 64 + 1 = 2^6 + 2^0$
$B = 66 = 64 + 2 = 2^6 + 2^1$
$D = 68 = 64 + 4 = 2^6 + 2^2$
$H = 72 = 64 + 8 = 2^6 + 2^3$
$P = 80 = 64 + 16 = 2^6 + 2^4$

Other letters don't share this property because

64 + 32 = 96 which does not correspond to any letter. The letter a begins at 97

For the newer hints

$0 = 48 = 32 + 16 = 2^5 + 2^4$
$( = 40 = 32 + 8 = 2^5 + 2^3$

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    $\begingroup$ Obviously moving in the right direction with the ASCII table. $\endgroup$
    – ElPedro
    Commented Apr 18, 2019 at 12:20
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    $\begingroup$ @ElPedro I guess Akari has formalized the informal property of my answer much better in his answer $\endgroup$
    – RedBaron
    Commented Apr 18, 2019 at 12:22
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    $\begingroup$ Still a good answer though :) $\endgroup$
    – ElPedro
    Commented Apr 18, 2019 at 12:42
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    $\begingroup$ I like the format of this answer more than the accepted one, simply because that's how I internally rephrased the accepted answer before even reading this one :) +1 $\endgroup$
    – Flater
    Commented Apr 19, 2019 at 8:24
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I think it's:

Each letter's alphanumeric value is double its predecessor, which also means, sum two times the alphanumeric value of the previous letter

This means that:

Starting from A=1 we get the sequence 1,2,4,8,16,... which corresponds to the sequence A,B,D,H,P

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    $\begingroup$ Welcome! Again, a great answer but not exactly what I am looking for. +1 all the same. $\endgroup$
    – ElPedro
    Commented Apr 18, 2019 at 12:14
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Each time you add the position of the letter (A is 1 and B is 2), the next letter's position is the sum of the previous +1. Thus, 1+2 is 3, +4 is 7, +8 is 15, +16 is 31. You can't continue the problem because there are only 26 letters in the alphabet.

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