Is Booming Blade's range relevant without a reach weapon?

Question

When looking at When does a readied action trigger on "when the enemy moves"? it occurred to me that I may not understand Booming Blade as well as I thought I did.

Here is my point of confusion: if an enemy is 5 ft away from you when you cast Booming Blade, you do not move closer, and you do not have a reach weapon in hand, does the spell fail or can you make the melee attack anyway? To put it another way, if you are out of movement and you don't have a reach weapon, does Booming Blade's effective range drop to Touch?

I think part of my confusion is that Booming Blade is not worded like most attack cantrips.

Booming Blade

[...] Range: 5ft [...] As part of the action used to cast this spell, you must make a successful melee attack with a weapon against one creature, otherwise the spell fails [...]

It's not a melee spell attack, like thorn whip, so I would assume that the attack here is held to all the standard restrictions that apply to a melee attack with a weapon, namely that you are attacking "a target within 5 feet of you". (Player's Handbook, pg. 146)

By contrast, I always thought that ranges indicated the maximum distance away a target could be. In that case, wouldn't an enemy that is 5ft away be at the range of Booming Blade (5ft) but too far away to hit with a non-reach melee weapon?

Can you successfully cast Booming Blade from 5ft away without moving and without a reach weapon, or no?

Answer 1

You can only cast Booming Blade on adjacent targets

In D&D 5e, typically "5 ft away" is considered adjacent to you. Note that your two definitions of the range are actually synonymous: "within 5 feet" and "maximum distance away" can both be mathematically represented as Distance < 5 ft. Anything more than 5 ft away is no longer within non-reach melee range. The specific wording of 5 ft away should be visualized as the length of a line between the attacker and the target. 5 ft represents how far a creature could reasonably strike by combining the length of the weapon and the minor posturings that are assumed to take place during the course of combat (D&D assumes that characters are constantly bobbing and weaving around attacks, parrying, etc.)

A grid would make this clearer

Using a grid makes this more apparent. The distance between the attacker and the target is 5 ft if they occupy adjacent squares, while 10 ft away would put one grid square (small or medium creature's space) between you and the target.

Different wording for different spell attack types

The 5ft range is used because a range of Touch is reserved for spells where the caster must touch the target with their hand (or other appendage). Since this is a melee weapon attack, the spell uses the explicit range of a non-reach melee attack in order to prevent reach weapons from extending the range of the spell (which would make the spell much more powerful when used against opponents who do not have reach).

Exception: Spell Sniper

Please review this link, which explains why the feat Spell Sniper will actually allow you to use Booming Blade with a reach weapon to attack an opponent up to 10 ft away.

Does Booming Blade and Spell Sniper Stack?