keeping spell slots "open"?

[Herobizkit]

For those who still use spell slots like RAW:

I recall reading somewhere that it is possible to leave spell slots "open" in order to facilitate memorizing the spells you need on the fly. What is the "official" or optional rule, exactly, and where can it be found?

 

[Goolpsy]

You can leave as many slots open as you like, and it takes 15 min (or 30 can't remember which one) to prepare spells in the number of slots you want.. etc. you can easily just prepare half your spells... late prepare 1-2-3 more.. and later you could preprare the rest if you wanted to.

 

[shilsen]

You can leave any number of slots open. Filling them later takes a minimum of 15 minutes and can take more depending on how many are left.

It's on pg. 178 of the PHB (bottom paragraph, left column).

 

[frankthedm]

The fighter/wizard and the late wizard of my game like to do this [wizard slain in unrelated event]. They would usually leave a slot or two open at each spell level and prep as needed, which was quite often thanks to the cruel amount of curve balls my games toss at the PCs.

A very good tactic for a game where the DM is on the ball and spelldumping is a real liability. But with a mollycoddling DM that let the PCs sleep where ever and whenever they feel like it, not worth the trouble.

 

[Shallown]

I tend to leave quite a few (at least one each level) when in town and a few while traveling when I play my 12th wizard. It comes in handy at times.

later

 

[dcollins]

3.5 SRD, "Magic Overview: Arcane Spells: Preparing Wizard Spells: Spell Selection and Preparation":

When preparing spells for the day, a wizard can leave some of these spell slots open. Later during that day, she can repeat the preparation process as often as she likes, time and circumstances permitting. During these extra sessions of preparation, the wizard can fill these unused spell slots. She cannot, however, abandon a previously prepared spell to replace it with another one or fill a slot that is empty because she has cast a spell in the meantime. That sort of preparation requires a mind fresh from rest. Like the first session of the day, this preparation takes at least 15 minutes, and it takes longer if the wizard prepares more than one-quarter of her spells.

Similar language under "Divine Spells".